I added two spacers made from 0.015″ brass shim stock to the collar on the paper clamp shaft, so that the collar would be flush with the step in the shaft and thus provide good support to the inner O-ring I had installed there.
As it turns out this is just a tad snug, so the paper clamp does not move freely. I will cut another slightly thinner spacer and switch it with one on the caster to loosen up the joint a bit, but other than that, this part of the work is done.
The paper clamp itself is not making a good seal against the ribbon, as the leather sealing block has hardened with age. I also foolishly removed it and in replacing it I think I may have installed it reversed, so the shape it has conformed to over the past fifty years no longer quite matches the new contact pattern. I have left it in the clamped position and I will see if the seal has improved tomorrow.
The next thing I have to come up with is a stand-in for the ribbon take-up reel. If there is no take-up reel to provide a but of tension on the ribbon, it tends to lift off some of the sprocket teeth, and the the holes still on teeth tear out when the sprocket teeth try to advance the paper because there are too few teeth engaged. As a result of this I already have a few torn-out spots on my test ribbon that I’ll have to repair with glue and tissue paper. Getting a take-up reel is in the works but in the meantime I may see if I can cobble something together. Some of the tear-out may also be the result of poor adjustment of the stops for the ratchet tooth motion, causing the sprocket to jitter a bit as the ratchet teeth engage.