Changing the buttstock on a '42 Lithgow, have a question about the screw

[SDWriter]

Only reason it has the square is to stop the bolt coming loose in harsh environments like war, which I suppose you don't plan to join with that rifle.

Well heck, if that's the only reason for the square head to stick out, then I can easily not worry so much. I assumed it must be to support the handguard structurally in some way. You make sense though, since the #4 rifles don't have this feature but instead use that spring-washer as a lock washer.