What's the difference between these wheels?

[Mart]

Those are 39-48 brakes, can't say if early or late without seeing the adjusters. Do you have an emergency brake?

To fit your flat face 40s steels, you should use 40-48 hubs and drums, which will fit on your axle. If you want to retain those drums, you should cut or grind down the 5 raised nubs to give a flat surface for the wheel to bolt against. The studs will be long enough - but only just.

Mart.