I can't give you a clear answer Adam, but you might look at this thread:
https://www.outdoorking-forum.com.au/forum/u...4&Words=greenfield&Search=true#Post26576

In conjuction with Keith's comment, there might be an answer there. Unfortunately although this problem comes up from time to time, we never end up with a picture of a properly set up Greenfield drive belt, for future reference. When you get this sorted, please post pictures.

Note that A138 as a belt size means an A section belt (1/2" wide on the wider side) that is 138" long, not 1.38 metres which is only 54.3". The common types of belt still have Imperial dimensions.