Daryl-

This one isn't a pinfire gun.
I have pinfire guns, rifles and shotguns, some very nice ones at that including a signed, addressed 28b Lefaucheux high grade gun from the Vivian road shop.

Note that even the early patent of the pinfire cartridge called for a rim. Not as much of a rim as modern centerfire shot cartridges, but a rim anyhow.

The early Lefaucheux action type utilized only one bite, like this gun does, later guns from the late 50's and early 60's almost all had two bites like the Jones pattern.
Did you notice that the action opening lever on this gun is the trigger guard?

As far as provisions for firing pins on the tops of the barrel chamber areas, this gun doesn't have the notches seen on pinfire guns. A Lefaucheux type pinfire cartridge wouldn't be able to chamber in this gun. The flash holes for ignition are centered about 3/16" downrange from the breech faces, holes surrounded by metal - no pinfire pin would be able to slide in and back out.
Note though that there are vertical keys on the action breech faces, similar to keys seen on some muzzle loading rifles which would allow for simple removal of the breeched barrel assembly -- on this gun they're pierced and the holes through them go through the standing breech of the action. Under the wood at the head of the stock there are pins through those holes which are held on to the action with thin strips of spring steel. Almost as if they were some kind of breech blow-back pressure relief valving. I doubt that was the intent though in that the gases would escape out against the stock head and blow the gun if that were to occur -- also the pins and spring steel retainers are held captive, inletted into the stock wood.

Interesting, dontchya think?

The reason I bring this gun up in the context of Curl's 12b rifle thread is that the action lightening cuts are of similar-to-identical style and execution of the ones seen on his gun. Hearing that Marrakai has a gun with cuts that look the same, and also knowing that Marrakai has gone to great depths with other guns of his to find the discreet origins of it's precious bits, I thought I might have a chance to learn something about my slick and slender little 20b SxS here.

--Tinker