The cyl. Lock Pin is seen at the rear of the frame, it is in a hole drilled half in the frame and half in the Arbor (base pin). It is the reason you can't unscrew the arbor. Drill out the existing cylinder lock with a tap drill suitable for a #6-32 screw, and with a new base pin in place, if the original can't be straightened, redrill the hole.
I don't know why the exploded drawing shown on oldsmokey's posting shows the locking pin sideways, it is not, it is parallel to the arbor. On an Uberti, the locking pin is the same part as used for the (#23) barrel locating pins. On a Pietta, they aren't.
It may, on a new base pin, be necessary to mill its half hole with a ball end mill, after marking its location. It would be a gunsmith's or machinist's job. Tap the hole after assembly for #6-32 and seat a set screw to lock the parts. With a set screw, future removal will be easier. Otherwise, a new pin would have to be driven in, matched to the diameter of the hole.
The wedge slot must be aligned and set in the correct position to properly fit the barrel lug. David Chicoine's book on Gunsmithing Guns of the Old West has dimensions for the base pin by Uberti.