Sorry I'm coming into this kinda late but there's one possibility I didn't hear aired.
Steve Maas wrote
>
> Hmmmm....lugged or fillet brazed, this is just the seat tube
> inside diameter, which should be the same at the seat cluster
> or below.
Not necessarily! In fact I'll go further and say not even likely, in this case. 25 mm (or 24.8) is so small that, if that is the inside diameter of the tube, AND the outside is a normal 28.6 mm (English) or 28.0 mm (Metric), then the tube would be so thick that this frame would weigh 15 pounds or more. Not even Schwinn balloon/cruisers had tubes that heavy. That seatpost diameter is not uncommon for small-tube aluminum frames (Alan, Vitus), which can have that thick a tube without being too overweight, but in steel? Never. Even if double-butted, that's too thick at the ends, no DB tubes have ever been made that thick, not that I know of anyway.
So what is going on? I'll bet it is a reducing sleeve that is brazed into the inside of the seat tube. This is actually a pretty good way to make a lugless frame. Single-butted seat tubes (the most common config) are very thin at the top, and make a less-than-robust seat cluster if not reinforced somehow. I've heard opinions to the contrary, and certainly there are examples of frames that lasted a long time with 27.2 seatposts and no lug. But I have seen more than a few failures with my own eyes too, so I know an unreinforced SB seat tube can definitely cause problems. Note, this is why Cinelli lugged frames use the odd 26.2 seatpost - they are reinforced from the inside AND the outside.
The bottom of the sleeve usually makes an abrupt shelf which can be felt from the inside, unless they made a tapered transition. The tapered transition is hard to do and not really necessary, so most builders just chop the sleeve off square. If the bottom of the sleeve is too far down to reach with a fingertip, you can probably fish for it with a spoke head or similar bent wire feeler.
Again this conjecture is all based on my guess that the seat tube outside diameter is normal 1-1/8"-ish (including 28.0 metric in that category). If some sort of undersized tube was used, then this could just be the unsleeved tube wall. It probably isn't 1"-ish (including metric 26.0 mm in that category) though. With an ID of 24.8, that doesn't leave much metal. And tubes between 26 and 28 mm are quite rare in bike frames.
So if I'm right, then feel free to ream, you're just removing metal from the sleeve, not thinning the tube. Certainly if the ears are pinched in from overtightening the bolt on a too-small post, the pry it out a bit before reaming, but don't overdo it - too much prying is more likely to break/ruin the frame than reaming from 24.8 to 25.0. Choose an experienced frame repair person or framebuilder to do the work.
Mark Bulgier
Seattle WA USA