I had misunderstood about the serial number on the yoke. The location you illustrate is the standard location for yoke serialization; you can find serial numbers there all the way back to the beginning of the 1900s. Sometimes you can read that number through a charge hole if you get a good sidelight on the yoke face/cylinder gap and if the number hasn't been buried in combustion waste.
On the other question, I think your front locking pin is fine. Looks to me like the center pin is 1/16" too short. If you go shopping for a replacement, don't buy just any K-frame center pin. The configuration changed at some point (in the '60s, I believe) and the later ones won't do what you want. I bought a couple of old-style K center pins from Dave Chicoine a couple of years ago, so he's a possible source.
