An update on the steel plates with three screw holes and a center 'guide' hole.
Both door hinge plates were intact and accounted for.
Both top frame to body captive plates were intact and accounted for.
I went through the whole car looking for any hint of a home for these. No luck.
What I know is these 5 things:
1. The three screw holes are about 5.5mm, or about 7/32 in diameter, with the threads not looking like a fine turn, or a very coarse turn, just somewhere in the middle.
2. My door hinge and top frame plates are in place.
3. The PO thought enough of them to wire wheel them.
4. I was able to find a left over screw from Victoria British marked 12-2356 that fit, from my spares box. However when I referenced this screw on their web site, no luck on the pages found, meaning it may be the same screw size, but probably a different length and hense a different number.
5. There are two of them, so I know there is a left and a right.
I've taken a closer photo of these two pieces, just in case it rings a bell for anyone.
Thanks again for any help!