That is weird all right.
Looks aside, it just seems to be a way to pick your bbl firing order without moving your trigger finger for or aft...just up n down instead..kinda like an instant bbl select single trigger, but not quite,Lol.
That would be tough to get used to after normal DT guns
I also reckon it could be to assist with some right hand injury or problem
Whats that on the rib by the standing breech face?
Looks like a fine Gun though
cheers
franc