Sorry for my ignorance, but what do you mean by "metric" tube sizing? Anything specified in mm would be metric, wouldn't it?
I have two 1980s French bikes in Columbus tubing, one SLX and another SL, one acquired new and the other having one previous owner, and both have a seat tube ID of 26.6mm. My three French bikes made of 531DB both have 26.4mm ST ID. There are other possibilities, of course, such as lighter-gauge tubing, or seat tube honing, creating a bit more room.
Last edited by mikestr; 10-09-23 at 01:49 AM.