Which Sub-Machinegun is this?

[lugerholsterrepair]

John, Having shot many thousands of rounds in open bolt sub guns, The bolt handle is of no concern!
I am easily confused..not your fault!

An open bolt does have the possibility of pollution no doubt but many sub guns have a cover for that reason.
One reason for open bolts is cooling. On fast firing subs heat can cause cookoff's so the open bolt helps to prevent them.