In a fascinating example of poor timing, disk images of OS/2 2.0 pre-release level 6.605 from July/September 1991 were missing for over 25 years, only to show up literally one day after after the 25th anniversary of the OS/2 2.0 release (big thanks to a very helpful reader!).
Let’s go back to the odd numbering for a moment. The build “levels” of OS/2 2.0 started apparently at 6.00 and went all the way up to 6.307 for the GA release (though the 6.2xx range may have been skipped entirely). The 6.605 pre-release from September 1991 became available between 6.149 (July) and 6.167 (October). It is not known what possessed IBM to assign it a completely out-of-sequence number. It is known that 6.605 is very close to 6.149 but contains additional fixes to work better with LAN Server (LS) and Extended Services (ES) pre-releases.
For reference, OS/2 1.0 used internal revisions 3.xx, version 1.1 used internal revisions 4.xx, and OS/2 1.2 used 12.xx. The fact that OS/2 2.0 used the 6.xx range probably reflects the fact that its development started after OS/2 1.1 but before 1.2.
Even though the 6.605 disks are clearly labeled 7/91 (July), they were just as clearly finalized in September 1991. It is possible that 6.605 was somewhat unplanned because the next level (6.167, or OS/2 2.0 LA) was the first to contain the Workplace Shell (WPS), and putting that together may easily have taken longer than anticipated.
The upshot is that 6.605 was in fact the last OS/2 2.0 pre-release which didn’t use the WPS and instead utilized the same old Desktop Manager as OS/2 1.2/1.3, which makes it the closest surviving relative of the MS OS/2 2.0 SDK:
The look and feel was not quite the same as OS/2 1.3 and in fact was a cross between OS/2 1.3 and Windows 3.1.
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