I haven't confirmed myself but have read that the threads are 4.5mm. A 5.0mm bolt definitely won't fit in mine.
As soon as I can find some suitable material, I will make a set of new base plates with campag outer diameters but the inner hole will fit the shimano boss, not the much more common campagnolo square boss.
I have measured my bosses and also the equivalent part on some shimano curved washers. The bosses are slightly smaller, so I'm looking for a spec sheet or something to confirm what the dimensions should be.
If you have a frame with shimano type B bosses, could you measure them? diameter of the circular part, total length along the long axis (diameter plus however much the tab protrudes), and width of the tab?