to be honest I remember them only from early 1990s catalogs, never had one in my hands.

this looks like a grip, I would say turn the barrel, maybe 90°, then its unlocked and you pull the barrel forward.

this part here in the red circle

looks like it cock the firing pin of the shotgun action by hand when you turn it upright.
maybe the 98 bolt action must be in the decocked postion so the common trigger operated the shotgun firing pin then.