Firstly, there is no "specification".
The rule is what works, works.
Suffice to say that there are many different quality belts, all vendors selling for what the market will bear. Some are garbage, some are good quality, and others are excellent quality.
I have been buying from this ebay vendor for many years, for my own and for customer repairs, and I have not had a failure or deterioration as yet.
His name is Fred at Marrs Communications marrscommunications@marrscommunications.com
Friendly, and fast service.
Menahem
Learn from the mistakes of others - you'll not live long enough to make them all yourself!