I just looked at an old thread I made when I was trying to figure out my BB situation. The inside diameter was 40 MM, but I was using a ruler, not calipers, to measure it so I could have been off by a mm or two. If I were a betting man I'd put my money on it being a thompson BB.

EDIT:
38 MM Thompson threadless BB.

EDIT #2 : You should look definitely consider the second link IthaDan posted. Those bottom brackets look about 100x better than Thompson bottom brackets, which suck.