BU

The BU was a case design used by Olivetti in the early 90's. It was used for many Olivettis like the Olivetti 290 SP, 486/33, M290-25, M290S, M300-04, PC PRO 33, PC PRO 290 SP, PC PRO SX 16, PC PRO SX 20, SX 16 and SX 20. For some reason, Olivetti decided to give multiple names to this case design even if it didn't seem any different from others... Which is the complete opposite of what they did with the XP 2655s, where they were all named "XP 2655" even if they were clearly different from each others... Eh.

BU 290 S-XX
It might've also been used for the Olivetti 290 SP.

BU 486 S-XX
This case "design" was used for the Olivetti 486/33.