There is strong evidence that at least the first lot of Garands in 1959 were those provided to GB under the LL program. The dates and specifications of the Lend Lease Act narrow the serial range to late 1941-early 1942 and rebuilds from earlier production. The story of how and when Cummings acquired Garands from the Britishicon government is well known (by buying Cogswell & Harrison as you state). There were almost no M1s in the US collector market prior to IA imports in 1958-59 except a few from DCM sales and vet bring-backs. The imports were in the right serial range and had the red paint on them, so we can be pretty confident they were the Lend Lease guns. After that first lot proofed under the op rod, we can suppose that M1s in the right range and with BPs could be LL, but we can't be absolutely sure.