Thanks Dean, I didn't even pay attention to that number. Was just about to lay the rims out and begin measuring the offsets.

That sort of figure makes sense but there is also a fair bit of dish for a 6 inch wide rim and there is another number on the back that ends in -1, so if you could clarify that that number on it's own stamped on the inner rim is indeed the offset figure then I'll start checking them anyway to see if they are +15.