Ricketts is a good Ebay seller who sells a lot of reloading equipment, including a lot of Star equipment. Generally knows what he is talking about, but in this case I would be surprised if he were correct in his assessment that the shell plate would be for a progressive only, and would not work on a Universal.
Progressives have a slightly smaller base, and many shell plates designed for a Universal will not fit on a Progressive. However, any shell plate that fits a Progressive should work on a Universal. I have had quite a few 38 Special shell plates. Most are unmarked, but some have a U stamped in the bottom. Star used the U on their parts list to designate parts that were unique to Universal machines. I have compared the shell plates stamped with the U and could not find any difference between them and the unmarked 38 Special shell plates. Having said all of that, I have never taken a shell plate from a Progressive and tried to use it on a Universal.
dverna,
333OKH is the guy to contact regarding converting a Progressive to 9mm. I know he was in the process of doing it, but don't know if he finished or how it worked out. I made a base plug for him so he could use a taper crimp die in the last station. The hole in the Progressive is much smaller than the one in the Universal. I know George also mentioned something about machining the 9mm shell plate so it would fit a Progressive.
Bruce