Kind of sounds like a Brewster indexer or a clone therof.  Sorry I dont have a picture, I had one at one time and got rid of it.  There may be a pic out there on the web with some searching.  Bruce is correct, there is a small block that is clamped onto one of the link arms, the block has a small rod that comes down and actuates the indexer to push the case and shell plate forward.  Good luck.