Sorry I don't have a part number for the belt I used and I haven't bought any for a number of years now. I used to buy them from a good old electrical repair shop who used to have them all hung up on his wall. He told me it was a commercial hoover drive belt, but there was no packaging or reference numbers. Mind you it was an amazing place and when I told him I was using them on my bike he would sell them to me for half price
I did have a look online to see if I could work out what it was, but no luck so far, I guess it may have been for an old hoover and time moves on. The shorter round rubber belts (usually sold in twos) make great tank rear straps. If I do find a reference, I'll let you know.
Regards Nigel