Weird trigger guard

[Guamsst]

I picked up a mismatched No4Mk1 Longbranch this weekend with an aftermarket magazine. The triggerguard is odd though. It has a square tab that protrudes into the magazine area and presses against the front of the magazine. Looks like a mod to make the aftermarket mag fit snug but it is done so well it seems like a factory build. Anyone familiar with this?

[Mk VII]

There are several varieties of wartime expedient trigger guard and it might be one of these.

[Guamsst]

There are several varieties of wartime expedient trigger guard and it might be one of these.

I think this will interfere with a standard mag though but haven't checked yet. I may try to post photos later.