Originally Posted by
juvela
Verktyg hath writ that following the fire at the factory there was a period where Bertin cycles were constructed in Belgium. If your shell is ~70mm that would tend to indicate a BSC threading.
I thought only Italian used 70mm bottom brackets. That said, I have one bike with Italian threads and a 68mm wide shell, so as has been pointed out the design is flexible. Based on the OP's cup measurement, I'd guess French threading, but it is definitely worth verifying that before trying to thread something different in there.
The nice thing about the IRD bottom bracket design is that the cups can be changed independent of the bearings and spindle (it's not really a cartridge but it does use sealed bearings).