I've finally finished this batch of machines, with a handful of other machines interrupting my progress, and good thing because I am due to head back to Minnesota next month for another pile...
The final machine was the Red Eye in the picture, which I wanted to convert from back clamp to side clamp. When I thought I was finished I just couldn't get the presser foot lever to lift such that I could check the tension release, the tension release lever was blocking everything. I couldn't understand how just changing the presser foot bar could cause this.
After about an hour of snarling (head banging on the workbench requires at least 3 hours of prior snarling) at the machine and checking two others to make sure I had the parts in correctly, I finally thought to check the tension release lever against one from a parts bag. Lo and behold the parts were different, even though the part numbers were the same. Although it seems evident that this machine had been worked on at least once before, I am still considering a strongly worded letter to the Singer Manufacturing Company.
I think the larger of the two is the newer one, looks and feels cheaper than the other. Anyway, once the switch was complete things worked as intended. I will wonder for some time though as to what machine the larger lever goes to. Maybe I'll never know.