[DonVoigt]

Yes,
it looks like someone removed the "artillery sight " from the barrel and cobbled some kind of receiver sight onto the rear toggle. You can get a replacement original type rear sight, but I have no idea what the non standard rear looked like.

I think you are correct, you have a weak hold open spring.

Firing weapons that have been in a "fire" is usually not a wise idea.

Welcome to the board.