The round doesn't keep the thumb out of the way, the thumb pushing down on the top round keeps it from moving; it can't move forward and let the bolt close until the thumb is removed.
Amsdorf, I note you did not answer the question about your own service or age and how you have such expert knowledge of what was done in those years.
I am not going to argue with you any more; you can have your own ideas and your own "proofs". But if you ever actually get to see one of those rifles, you might read what it says on it. It doesn't say "GARAND", it says "M1".