Just to clarify, I was aware of the two simultaneous updates and did not attempt to install the old one. In fact, it was the reverse, something that no one else is likely to attempt. I was working on my laptop when I read of the update. The error message that I received occurred this way:

1. tried to install update for build 327
2. error message said I needed version 2011.5 to do so
3. started BIAB and discovered (Help, About) I had not yet installed the download for 2011.5 on this laptop
4. exited and installed 2011.5
5. tried again to install update for build 327
6. error message told me I STILL did not have 2011.5 (but I did)
7. started and ran BIAB, exited
8. tried the patch again; worked this time

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.