Audio craft AC 3300 on fly VTA

It looks like the arm comes in two flavours with and without a micrometer. At the same time it looks like both have the same model number.

The AC-3300 comes with micrometer and the AC-3000 without it.

Was the micrometer an optional accessory and what is the proper model # for it?

AC-3000 can be upgraded with micrometer by replacing the center block model # CB-3040.

Mine 3300 came without the micrometer so I suspected  that somebody replaced it with the base from 3000.  Thank you  for the center block  model. This is exactly what I was looking for. 

CORRECTION:

Actually, the AC-3300 also comes with a lightweight arm base option for suspended table, the arm base model # CB-3040LB. Perhaps your arm is equipped with the lightweight arm base.