Thanks for taking the time to do this. You might note that the Sellier & Bellot ammo is from the Czech Republic, not Russia. I was reloading some .303 last night and noted that the IMR 4895 powder looks exactly like the IMR 4064. The powders have different burn properties, so I suspect that even though the powder looks identical in your bullets, its probably too much to conclude its actually the same type of powder.
Can you measure the bullet diameter on yours? I;d be interested in knowing if they are all .311.