GI Bob,
I'm leaning towards the WRA plate on the CCC chart, it seems to match up best row by row.
2nd guess would be the Inland/SG plate that btw I just realized is the same picture... If you look closely at this example at a scratch or dent you'll see it on the other example.
The U.S. Caliber .30 Carbine Buttplates
I always like to credit Jim Mock for his work on putting all this info together.