Similarly on guitar straps
You can buy both new repro style and original GM belts from the States that are in a similar profile and size as that used on that guitar strap. In some cases the belt and guitar strap buckles are genuine original buckles that are being sold in new markets, clothing accessories etc.
The 64 - 67 Chevelle had that small type belt buckles and these are very "similar" to the GMH ones in the some Torana's and other Holdens.
The main difference (other than the Australian Safety markings) is that the GM buckle is "back to front" to the Aussie one.
With the GMH belt the clasp pushes into the "flat" end of the buckle, whereas in the GM belt the clasp pushes into the "sloped" end of the buckle.
I have seen photos of a set of belts made by a prominent Australian seat belt repairer and they have used the a buckle like the American GM belts and I'd say there would be no problems thinking they were the correct GMH belt, especially if the correct centre label was applied.