A bit better focus on my part brings up the apparent fact that I'm
full of $_!_ about it being a H&R 155. The 155, along with the Handi
rifle, uses a push down action release lever that's located to the right
of the hammer. OP's rusty relic has a shotgun style tang lever on it.
DUH. Talk about overlooking the obvious! Oh well--disregard most of
my previous post.
ETA: Well folks, I'm going to redeem myself for my previous stupid.
The OP's gun may be a Harrington & Richardson line throwing gun. My Google-Fu
is STRONG today! The only oddity is the listing specs a shorter barrel. Maybe
they made a couple of variations?