Contemporary 5T Pre-Unit Triumph bikes have a 63mm bore, so surprisingly Triumph pistons will fit a Longstroke 62mm + .040"/1mm bore, even down to the gudgeon pin diameter, give or take the odd thou. From research I reckon some sellers list these two pistons as being the same, and even muggins thought he had found rare Longstroke pistons that turned out to be everyday Triumph. As always, there are variations in compression height of any pistons offered.
Standard 63mm Triumph rings will fit Plus 40 BSA Longstroke Pistons which also have ring thickness 1/16" /1/16"/1/8".
Not much help in this case, but maybe of interest.
eBay is probably the best source for choice, even contacting sellers directly rather than waiting for a listing to appear.
Swarfy.