The OS/2 Museum site said that the finding was a fascinating example of poor timing. The 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 the 25th anniversary of the OS/2 2.0 release.

This was pretty much what you would expect from OS/2, which suffered from problems like this for all its life, and was eventually killed off by some evil Microsoft skullduggery.

The last OS/2 2.0 pre-release did not use the Workplace Shell (WPS), but "instead used the same old Desktop Manager as OS/2 1.2/1.3”.
Historicially it was the closest surviving relative of the Microsoft OS/2 2.0 SDK. It featured a 16-bit/32-bit hybrid kernel and a "DOS Window" icon (as well as a few games like Reversi and Klondike Solitaire).

It did not have the same look and feel as OS/2 1.3 and in fact was a cross between OS/2 1.3 and Windows 3.1.

The elusive 6.605 pre-release fell between 6.149 and 6.167 -- and "it is not known what possessed IBM to assign it a completely out-of-sequence number.

“It’s probably fair to say that OS/2 2.0 level 6.605 from September 1991 was the last pre-release very similar to the MS OS/2 2.0 SDKs, both visually and technically. The next pre-release, 6.167 from October 1991, looked and felt very different and was much closer to the final GA release from March 1992”, the site said.