Not wishing to totally hijack Timothy's Camaro thread, but also because I think it's worthy of standing by itself, I'll repeat the tip for checking whether the reference number matches the case material here in its own thread (along with a bit more info on the gold used in the plated models thanks to Arno's post):

For everyone, there's a helpful hint in the model references for a while around the end of the 60s and through a large part of the 70s. It's not a hard and fast rule by any means but it might help if you're on the spot and faced with something unfamiliar:

Reference ends 3 - case is stainless steel

Reference ends 5 - case is 16K 20-micron gold-plated. Caseback is steel

Reference ends 8 - case is 18K gold. Caseback is also 18K gold

It's a loose rule, granted, but if it helps someone make the right decision or better yet avoid the wrong decision, then I hope it's worth pointing out! Earlier solid gold Heuers also used 14K gold but by the time this numbering convention was in place, they should all be 18K. Some earlier Heuers were gold-filled (a different process than electroplating) but this had dropped out of use (by Heuer at least) by the time these references were used.

Mark