Very Nice indeed.

The narrative states they are chambered
for a "custom" 1 1/2" .50 cal. casing.
I assume they are central fire. If Rimfire, they
are close to the .50 American Peabody at 1.456".
The .50 Remington M71 was 1.25".
The .50/50 Maynard was 1.370".
The .50 Carbine was a shortened .50/70.
Since the .50/70 was 1.75", & the Carbine was shortened, perhaps the .50 U.S. Carbine or similar is the case used.
I find no listing in English nor European ctgs. that are close.