Last week, and ahead of the official launch of Windows Phone 8.1, we reported that Microsoft had reached the RTM milestone with its OS update, and that its code had been finalized ready for distribution to Microsoft’s partners. But it turns out that things are a little more complicated than that.

As we reported, Microsoft did indeed sign off on the Windows Phone 8.1 build – what the company refers to as “the final feature build” – on March 26. On the same date, that build – 9651.12349 – was made available to Windows Phone partners via Microsoft Connect.

But that wasn’t actually the ‘final’ build; indeed, WPCentral pointed out that a few bugs still needed to be exterminated in the OS, suggesting that this process was likely to take a couple of weeks.

An email shown to Neowin today, sent by Microsoft to its Windows Phone partners, now clarifies the timeframe that the company has set out for completing development of the launch build of 8.1.

The email refers to the remaining dev work as the ‘Final Adaptation Kit build’, which it says is intended to address “a small set of issues to improve the commercialization aspects of the product” (sure sounds like bug-fixes to us). Work on this is proceeding on schedule, and is expected to be completed on April 8.

The ‘final build’ from last week, combined with the more recent Final Adaptation Kit, will form the ‘real’ gold code for the OS – referred to as the snappily named ‘Windows Phone 8.1 QFE1 Final Adaptation Kit Build’ – which Microsoft will seed to its OEM partners. This code should be in the hands of manufacturers on April 14, before its rollout to existing users begins later this month through the Windows Phone Developer Preview.

The first new devices designed for Windows Phone 8.1 are not expected to go on sale until next month. Among the first handsets will be Nokia’s Lumia 630/635, while its new Lumia 930 flagship will go on sale in some markets in June.