Nice close up images. Thanks   Probably didn’t explain it clearly. It’s a small round mark on the rim — like a tiny punch mark. All McCord shells from lot 25 through 623 have these marks, while Schlueter shells do not. McCord shells produced after this range have depressions on the outside instead, including fixed-loop shells from 623 to 796. These can be difficult to see or even feel because of the paint and texture. It’s a quick and useful way to determine the approximate lot stamp range when the stamp itself is hard to read.   Dimples.   Dimples from the outside. Not all are this clear.     Depressions from the outside.    

USMC M53 (1953) HBT UTILITY COVER IN DETAIL    

USMC M52 (1952) HBT UTILITY COVER IN DETAIL    

USMC M51 (1951-1952) HBT UTILITY COVER IN DETAIL    

USMC M50 (1950) HBT UTILITY COVER IN DETAIL    

USMC P45 (1945) HBT UTILITY COVER IN DETAIL    

USMC P44 (1944) HBT UTILITY COVER IN DETAIL    

No Mikie, they are watchamacallits, fastened to the kanoogabubble with a murphypin. 😁

Dang, I remember fighting with one of those contraptions many times in the late 1980s..

Another photo of the display case dedicated to Lt. Robert F. Toner.  Not sure why my first photo posted upside down but this is another view of it.