Adam,
Quite a while back I purchased a Shape-Rite from another PM member who happened to be local (I live in a machinery desert). Mine appears to have been used in production, was crashed sometime in its life, and has a brazed tool head slide repair that seems quite strong.
Parts for these are pure unobtainum, BUT, at the moment eBay seller "
kwktools" (no affiliation with me) has several Shape-Rite parts listed (door, ram clamp handle, saddle). I purchased a ram stroke pointer from him (mine had the tip broken off) and the transaction went smooth, meaning reasonable price, shipping was fast and the item was as described.
This particular part isn't listed, but I think it would be worth your time to contact him through eBay and ask if he has the rocker arm. From what I can tell, he is parting out a Model B.
Here's a link to his currently listed items:
https://www.ebay.com/sch/kwktools/m...hRsAAOSwjthcWlBF&rt=nc&_trksid=p2047675.l2562
By the way, do you have the later "Model B", or, like mine, the earlier "HY DUTY". Both are 8" stroke.
Good Luck,
Tenaya