I can't vouch for the black 1531 (the Plus/4 model, 1530 is the VIC/64 and they're all called C2N after the first, square one) but I have a bunch of broken 1530's of which I opened one.
The belt from motor to tape is approximately 26-27 cm in diameter, unstretched. The belt from tape to counter is approx 19-20 cm. Neither is stretched a lot. I would believe the mechanism in 1530 and 1531 is the same, only different interface.