'Scuse my naivete here, but wouldn't a seat post sizer tool like we have at our shop answer any questions?
I've used the seat post sizer tool on my own stuff as well as random mystery bicycles that people bring in. Not too long ago it helped me with a no name lugged frame - ordered the closest size seatpost and added a shim to it for a perfect fit. We'd have been guessing without.
Now if there isn't a reason to use one on an old frame, please tell me before I mess up someone's treasure!
Ann Phillips Atlanta GA - where it was 15 degrees Fahrenheit this morning - who says that hell is hot?