Thanks for the help guys, most educational.
I had a chat to the parts guys at Moorooka Yamaha who I've dealt with before, and he is pretty helpful. He basically confirmed what's been said here, ie the middle digits of the part number denote the part, the prefix indicates the model it was first used on. A lot of parts are interchangeable on this basis but not all.
He did offer some clues which I've put into use already. In my case I was looking at spokes for a YZ125F. I checked a couple of different Yamaha MXer's of similar vintage and found that some of the 250's and even 400's have the same hub part numbers and rim sizes. The spoke numbers were slightly different, but if anything would be thicker rather than thinner. Anyhow, a bit of research paid off, and instead of $120 per wheel I'm now looking at closer to $46.00.
Thanks again for the help.
