I had the same thought as you John, rivet holes from an Indian single shot conversion. But did RFI not use a wooden plug for the well secured with two wooden dowels through the forearm woodwork, the ends of which were visible from the outside as round dots? If I am thinking about the same arm, they used regular 303 brass with the neck opened up to make a straight wall case. The cartridge was loaded with a round lead ball, and the bore was opened to a larger diameter and smooth like a shotgun was it not?
More than one type of .410 shell. You are correct that an un-necked .303 case was used.
Cartridge SA Ball, BL,SB,.410 inch Cordite MkI, approved for Indian service January 1929
one round lead ball 10.4mm in diameter weighing 95gr, ahead of two thick felt wads.
Cartridge Sa Segmented Shot .410 inch Cordite Mark 1 approved Feb. 1929
twelve segmented shot contained in a bag.
Cartridge SA Buckshot, .410 inch Mark 1*
approved Indian Army L.O.C para 1042, june 1930
Eighteen buckshot with one glazeboard disc above and a millboard disc below
17.5 gr of Cordite size 1, above which was a glazeboard disc and a felt wad.
There was also a .410 ball drill round,
A .410 ball dummy round
and a .410 buckshot dummy
There are also TWO single shot .303 conversions, and yes they also plugged the mag well on the second type.
The first type had NO mag well and the trigger guard was a straight shot to the king screw.