Rick:
Thanks for that. It appears the end cap was an additional claim for the patent and used a roll-crimp:
It may be desirable in certain conditions, to provide an alternate means for retaining the shot in the casing portion of the shell FIG. 8 shows a shot retaining disk 31, made from a hard plastic material. The periphery of disk 31 is provided with a raised lip 33. The top portion of disk 31 is provided with grooves 32 across the diameter of disk 3!, which divide disk 31 into four quarters. FIG. 7 shows shot retaining .disk 31 crimped at 33 by inturning the end of the shell casing over lip 33 of the disk. When the shotgun shell is detonated, disk 31 will break up or peel away at the grooves 32 when leaving the muzzle of the shotgun without interferring with the shot pattern.
Frankly, there is no much new here and it took the PTO 2 1/2 years to get around to granting the patent. I don't see anything in the patent that would not have been obvious to one skilled in the particular area. I don't see any use/advantage for the screw on base; hence it would be unlikely anyone would try to infringe the patent (as history has shown). A waste of patent attorneys' fees.
And that thin line at the top of the hull that Gort focused on, above, appears to be a line that demark's a change in the thickness of the plastic. It's apparent from the drawings in the patent.