Interesting, the speed cap.
Never seen one, but in your analysis you say putting the old cap on solved the problem. To me that says as you theorize that the seat on the new cap is too high. I'm not aware of a shorter LCL lamp available for the fixtures so that means either they are seriously out of adjstment based upon the three screw lamp bases when they go out of bench focus, or there is something odd going on here. I would say call Altman, ask for Jay if possible, he is one of the "good guys" there.
http://www.altmanltg.com/
Explain your problem and the putting on of the old lamp base solve and see what he says. Than post it because I'm curious.
Wonder if it's possible you have Shakesphere caps on the fixtures and given a new design they don't sit in the same place for a good bench focus. That would certainly explain the problem though I have not played with the new Altman fixture to know for sure.
Send me a E-picture of the lamp base otherwise, I'll look it up on my parts disk or catalog from them and verify it. Otherwise, what would even be easier yet is if you went to the website and looked at their parts manuals for the fixtures and compared your base to that of the Shakesphere. What they have available on-line is very complete.
Might be able to find the proper lamp bases on E-Bay or another resale website either by someone selling them or you posting a request. Could be cheap enough.