This build is a Windows Vista B2 compiled in the VBL_UX (User Experience) Lab. It was released to technical beta testers, as well as certain coprorate customers by Microsoft, having been used as a testing board for the extensive feedback from the earlier Feburary CTP (Community Tech Preview) release.
They also described this release as an "External Developer Workstation", with the intention of providing an interim build between the two CTP releases.
According to Microsoft, they were still to release the next CTP release within the second quarter of that year, not to mention that this was to be the last build within the Beta 2 fork.
As far as functionality was concerned, this build included some minor user interface changes, most notably improvements to the Media Center, but also new Aero and Aurora effects, a faster Setup process, some new Sidebar gadgets, and slight improvements in overall performance and stability. Paint was also updated with the adition of a new screenshot snapping/saving tool, and even a slightly redesigned Network Center.
This build also did not meet CTP quality measurements, and according to Wikipedia, was also only available in the Ultimate release for both, 32-bit x86 and x64 (x86 64-bit) systems, however, since there were Server releases of all of the 5xxx series builds of Longhorn, where the Client counterparts were later given the name Windows Vista, this apparently means that only the Ultimate variant was actually released, as opposed to the other variants and counterparts, and not as to whether they were compiled.
The screenshot has been posted by Stephen Chapman on Microsoft Kitchen. <gallery> Image:5342.vbl_ux_partners_winapps.060316-0000.jpg|Desktop with several apps open </galllery>
