The pre 1913 LC Smith design used a bushing on the face of the breech to hold the firing pin in place. You unscrewed the bushing with a spanner to remove or replace the firing pin.
The back end of the firing pin hole inside the action was made so as not to allow the pin to drop back inside the action.
After 1913,,the bushing was done away with and from there till the end of production and even the early 1970's short run LCS made by MArlin, the pins are simply dropped into the action from the rear. No springs.
The assembled lock when placed in position in the stock holds the firing pin in place from falling back out of the action. Some that are better inletted will have a small shelf of the wood inside to keep them from doing that but most do not.
Regarding the 2 piece firing pin,,if you are going by the Numrich illustration,,I'd discount that for your shotgun.
They show retainers at the rear of the firingpins also,,so a completely different set up. To tell you the truth, the SxS 12ga Meridens I had,,IIRC had one piece pins.
Unless the Numrich gun needed two piece pins to get around a problem of perhaps not being a straight line strike,,I can't see the need. Off set slightly they do look in the pic and I think that may be the reason. The Meridens are all over the place in construction.
I'd make the pins for your gun as one piece, w/a protrusion of .045" or so. With some playing around with the length AND diameter of the back portion you should be able to find the right configuration needed to be captured by the lock bridle or plate so it doesn't fall back.
Even a wooden mock-up can be useful to get rough measurments. You pretty much have what you need in your drawing.
Don't make this more complicated than it needs to be.
