Stock question
 

Is there any reliable way to tell the frame size an orphaned buttstock was made for?

Accurate width and height measurements at the head of the stock is a good beginning, although no two can be expected to be exactly alike in both dimensions.

The Parker Story, page 527, displays a chart of the dimensions of all known hammerless frame sizes.

If the serial number is in the records, that will at least tell you the guage of the gun. You can narrow it down to within a few frame sizes based on that. Then do the rest with measurements. In theory, a given frame size should be around the same dimentions regardless of the guage. So for example, a No. 1 frame 16g. should have the same dimentions of a No. 1 frame 12g. But as Dean said, there is variation.

Page 527 and a vernier caliper.