Flat audio belts

[Turnip]

Hi Folks,

Having quite a few decent cassette machines in where finding replacement flat belts is a bit of a problem.

Seems that suppliers measure them in several different ways - Folded length - Cut length or diameter, which is difficult as most don't state which.

But seems size codes exist - AVXXXX or BXXXX Where if I knew what they represented might get an idea.

Looked about but could find no clue - Any thoughts ?  - Chris.