First models had twin blue bushes. First prototype of proflex system used an only small yellow elastomer. It broke easily so in the production series of 1991 they started using the two blue elastomers. In 1993 proflex models started to use a bigger mcu yellow elastomer if I'm not in a mistake, maybe in 1992 but I'm not sure.
RF-1 models weren't in catalogue. They were a very small production of frames supposed to be the race frames of the team. It is said that Serotta welded the rf-1 frames, so nowadays I'm not sure if actually were the RF-1 alu models or another model that was in catalogue wich was made of good steel with cook bros and other nice stuff the frames that Serotta really welded. (This model was named PROFLEX USA Limited)
IMHO It has more sense that the USA limited edition were the frames welded by Serotta.
(you can see the two blue bushes)
I had the luck of had a conversation with the PROFLEX Sales manager BITD and he told that few frames were made by serotta, but he couldn't tell me how many were made there and he wasn't sure if they were the alu RF-1 or the steel USA Limited. We were talking about some cranks they made wich were similar to the bullseye ones. He told me they only made about a hundred of them but I've never seen one of them except for older bw pics