Seeing that the hysteric plate has four nice screw holes you may as well use them. I figure there's not much of a market for stolen historic plates so just use some small button headed allen bolts.

No need then for any extra fittings for the back plate either.

Cheers, Ian

How neat & tidy does that look .

Just thought I'd update this for anyone who is interested. I bought the pair of number plates from the below seller:
http://www.ebay.com.au/itm/291553570278

And mounted to the car with some bolts:

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With two large black ares just begging for a pair of appropriate period stickers.....

Cheers, Ian