Thanks. Yeah, I've just cleaned up the light powdercoat "leaks" at the edge of the tube, didn't ream any amount of steel in fear of weakening the tube, but there's been plenty of reaming before me by the looks of it.
Right, loaded with the right amount of wine to waffle on. I'll do my best to factual to start with, then move on with suggestions.
First up, the time period - both our bikes are from the 50s. 50s bikes had
steel seat-posts in the "candle stick" format with a bolt on saddle cradle at the top. Seat-posts were short, and only about 50 - 80mm were sticking out. This is worth remembering.
Second up, what I found trawling tontonvelo is that the outer seat-tube diameter was kept standard at 28.0mm. Gas pipe tubing meant either a 25.4mm / 1" or a metrification to 25mm seat-posts. Lightweight tubesets were
pretty much universally at 26.2mm seat-post.
Third up, from my own experience wrestling in the asylum with Eau-de-Nil, I found that all the clamping is done at the seat-lug with a very narrow and fine pinch area, to the detriment of someone hamfisted. I observed a few other things. I'm actually not convinced the whole thing was reamed and secondly at least in my case the seat-tube "opened out" with a larger diameter once I got the seat-post in past the initial goose neck so to speak. Moreover, there is a wash of destroyed light weight posts on the web, crimped, bent and fit for the bin. Also found that the lug edges were extremely sharp (which I filed down) and I would even go as far as saying the French may have exploited this deliberately to actually (and lightly) crimp, score the post and help lock it into position.
Fourth up, there have been oddities of course. Vintage French shims are about. In fact I had one in my hand today. They are very short (40 - 50mm) and god only knows what diameters they are for, because nothing is marked on them. Sound familiar all this?
You can forget hang-ups like I had coming from the MTB side that a shim should be 65mm - 80mm - because the seat-post post by design should be made of steel with a virtually sod all extension
Chapter one over. Solution mode.