Very few beer brands survive today that have modern examples to put into a worthwhile four-decade vertical tasting. That’s simply because forty years ago there were hardly any beers being brewed that had the longevity to be still drinkable when even the most junior brewer involved in their production is now at or approaching retirement age.

It wasn’t looking good for Courage Imperial Russian Stout, which was one of less than a handful of strong beers capable of great age being brewed in the 1970s and which stopped being made in the early 1990s despite a history going back more than two centuries.

But Courage IRS, doubtless in considerable part because Michael Jackson’s World Guide to Beer in 1977 featured it across two pages, has inspired a huge number of imitators in the US and created an extremely popular beer style in the process.

When the Bedford brewer Wells & Youngs acquired the rights to the Courage beer brands from Scottish & Newcastle in 2007, the first two beers from the old Courage stable Wells produced were the Best Bitter and Directors Bitter. But I am sure it quickly occurred to the company’s marketers that here was a chance to bring back a truly iconic beer, which would surely have an instant appeal in the US as the ur-IRS, the Imperial Russian Stout in honour of which all others are named.

Thus in May last year the Bedford brewery produced the first new brew of Courage Imperial Russian Stout for 18 years, two bottles of which they’ve been kind enough to send to me, to my great delight, as I love a good IRS. And because I’m the sort of sad nerd who stuffs bottles of beers away for decades, I was able to pull out examples of Courage IRS from 1975, 1985 and 1992 to compare against the latest version.

The 1975 version, made at the Horsleydown brewery by Tower Bridge in Bermondsey, South London, actually has a bottle cap that says “Barclays Russian Stout”, reflecting the original brewer of the beer, Barclay Perkins of Southwark, which merged with its near-neighbour Courage in 1955. It opens with a slight “pfft”, always a relief with old bottled beers, and there is an instant aroma of chocolate, meat and coffee. The beer is totally black and pours with no head, but a fair amount of condition is apparent in the mouthfeel, and it’s entirely drinkable still after 37 years, rich and full, though without much complexity. There is a burnt malt bitterness, little trace of hops, and sweet chocolate comes through at the end. You would not guess this was a beer almost four decades old: it would still be a fine match for a chocolate dessert.

On another 10 years to the 1985, brewed at the John Smith’s brewery in Tadcaster, Yorkshire after the closure of the Horsleydown brewery in 1981. There is no sound on taking the top off, but a slight petillance shows up in the glass. The initial nose is of liquorice and dark brown toffee, and the colour is extremely dark brown, rather than black. The mouthfeel is thinner than the 1975, and the flavour is baked raisins and burnt batter: indeed, I found I was thinking of Garibaldi biscuits, which, by coincidence, were also once made in Bermondsey. This is less bitter than the ’75, but still remarkably drinkable for a 27-year-old beer.

Seven years later, in 1992, IRS was still being made in Tadcaster, though this was the penultimate year of brewing IRS by Courage. Twenty years on, it has an almost whisky-like nose, and the condition has again very nearly disappeared. The colour is back to black, and the mouthfeel is oily and sharply alcoholic. Sweet chocolate is still the loudest note, but underneath are bitter orange and, again liquorice. This is a lovely, heartening, rich beer and quite astonishing for its age: very enjoyable.

Before I got on to the resurrected Wells & Young edition, I was lucky to have another revived version to try: Ed Wray of the Old Dairy Brewery in Kent sent me last year his “cloned” version of Courage IRS and, fortunately, I had decided to wait a few months before drinking it, which meant it was around to be included in this multi-decade test. Ed’s IRS pours a lovely creamy head, and cream-and-chocolate were the major initial attributes. This was not, on my brief encounter, a particularly complex beer, but very enjoyable.

The W&Y IRS, made in May 2011, also pours a great head, but before that comes a massive hit of hop aroma on the nose: this is a beer that smells and tastes as if it has been rammed full of English hops (Fuggles was my guess, though I see from Des de Moor’s tasting notes that they were actually Styrian Goldings, which are, of course, despite their name, Fuggles-grown-in-Slovenia.) The first bottle, I was struggling to cope with the bitterness (I’m not a fan of strongly hop-aroma-forward, bitter-flavoured stouts), but as Stan Hieronymous has wisely pointed out, you can’t judge a beer properly on just one glass.

A second bottle the following day, by which time I was prepared for the hops hit and able to see over and through it, allowed the complexity to come through, revealing a beer with a suggestion of a hoppy Crunchie bar livened by a touch of ginger: chocolate, esters, sweetness lurking under, to me, an approaching-DIPA level bitterness. (Michael Jackson’s Beer Companion from 1993, incidentally, says Courage IRS had “around 50 units of bitterness”, which would give it, at 1098OG, a BU/GU (bitterness/gravity) ratio of .51 – far below the .90 ratio that the American homebrew guru Ray Daniels apparently believes is correct for the style. Hmmmm.)

The last time I drank Courage IRS anything like this “new” was in 1994 in the Forester in West Ealing, one of the few pubs selling the beer at the time, which had the 1992 version. I don’t remember it being then as noticeably smack-your-nose-with-hops as the W&Y version is: maybe it’s just me, though, as a quick check shows no one else tasting the beer seems to comment about the head-in-a-hopsack sensation I was getting. That (to me) over-hoppiness will, I feel fairly confident, die down over the next year or two, making this a beer I really, really want to come back to about May 2013, and again in 2014. And, ideally, in 2021.

Wells and Young’s head brewer, Jim Robertson, apparently set out to recreate the Courage IRS he remembered from the start of his career as a brewer, and it looks quite likely that he has succeeded in recreating a beer with the same massive potential longevity. This is, of course, just how it should be: the first mention we have of Barclay Perkins’s beer “specially brewed for the Empress of Russia”, from 1796, said it “would keep seven years”. If the W&Y version is indeed like the Courage one, at seven years it will be just an adolescent: it looks set to be a fine beer still after 20 years in bottle, and perfectly drinkable at almost twice that age.