I have a couple of spare buttplates that I got for my 20 ga, from the afore-mentioined repro supplier. One has the horizontal splines, 3" centers and was a bit dimensionally challenged. The other is the 'dogshead' which just makes width at its widest point, has a bit extra on length at 2-3/4" spacing.
Luckily, I did order an extra of the dogshead, one of which went on my Field grade (40's era). This extra dogshead will need a bit of flattening on the mounting face to correct the manufacturer's cleanup. It will be close on width. There just wasn't any to spare on the one I used either. However, it is better than the one that was on the 20 ga. with its shattered tip.
I think I got new screws to go with these also, which you may or may not need?
Even better, we are at least on the same side of the same state. Might be a decent chance to line the thing up for a test fit..
Too bad the darn things weren't better made with a fit allowance.