Most ZIP codes in the US are fairly familiar - 5 digits, like 90210.

However I've come across some weird ZIP codes in Anchorage - like 995MX. (Visible on this map)

Many websites (like google maps) don't accept this as a valid ZIP. Even clicking on it on the map above claims it's a "filler"? Where did this come from? Why are there these unusual ZIP codes around there, and do they have a 'normal' ZIP code alias?