The USAK American Knight and LK-93 Wolverine schematic doesn't show the "belly buttons" in the drawing. The Bighorn and up rifles all have them in the schematic drawing.
The Reference number is S18 for the Stock Bushing. The Part Number is 010202.
The technical name and numbers is as follows:
S18 010202 Bushing, Stock, .250 Blued
Probably going to have to call Knight, but according to the period correct Instruction and Safety Manual, it was never in the American Knight. So, you probably didn't lose it. If the stock hole is tapered/countersunk like the mating surface of the screw, it never had one. See below.
American Knight
Bighorn