baddboybill said:
I usually always order one size up and one size down just to be sure

But Royce I used FBM on my M70 with no issues
Bill, You are smart to order one size up and down. That is good!
I learned my lesson the hard way.
The Medium size width got a range. It can vary from 4.0 mm to ~6.5mm. The one I recieved was 6.5mm and that slipped off the M70 pulley in seconds. I'm sure you got the one close to 4mm width.
I bought them from studio electronics.