Need help on Alt belt numbers? New and improved? (x-posted to S-car list)

[Jerry]

OK guys, the wagon threw an alternator  belt while I was having fun testing her with the new MSD spark box installed. Pulls like a freight train again with the RS2 EC. 
I have some spare belts for my 3b in my collection. The parts places  now list 11x820 replacing 10x825 for the alt belt. It looks like the new belt is 5 mm shorter if I understand the naming convention.
I have a Conti 10x825 and a generic 034-903-137A which appears to be the same.  Can I use either of these? What is the difference between the old and new belts? Thx
Regards,
Jerry
91 200tqa