Just checked the part number with Tracy at Cosworth who agrees with me but, as a double check has asked a fella in the spares department who actually gave the items to teh fitters back in teh day when these were new and were being put into the cars. He says it has always been teh same belt fitted and sold at Cosworth and they have never sold anything else. The Part number in question is YBO200 and has been for over 21years. It is even numbered as an old number to back this up...

For example...
The Belt Guide washer is a YB0198 and the belt pulley is a YB0197. Proving these numbers were designated right back when the engine was first designed at Cosworth around 1985.

My thoughts on this are simple... if Cosworth thought that was good enough for a cambelt, why use anything else?

The Gates 5248XS is allegedly exactly the same belt and ive fitted them for years and years.