My experience with trapdoors started when I went to a gunshop and they had a nice looking one for a ridiculous price. I bought it and some ammo and started to check it out. The cartridges were too small for the chamber. I took it back to see what was going on and the gunshop owner told me they made a mistake. It was a 50-70. He then went into the back of the shop and gave me a trapdoor 45-70 without the lock mechanism.
He told me to go home and just switch out the parts. I now had two for the price of one. I just had to find a lock for the one I wanted to shoot.I went to Dixie Gun Works and Turner Kirkland fixed me up with a new lock. Everything was working just great. The only problem was the 45-70 was in so bad of shape that I sold it for about 200. I kept the 50-70 but I never put the cartridges together in order to shoot it.
You came out ahead on that one. A 50-70 is a fairly rare find (at least in as nice as a condition as you describe).
Here is a suggestion if you want to fire your 50-70.
Contact the folks at Friendship Indiana in the HQ of the National Muzzleloading Rifle Assoc. Though not affiliated with the NMLRA, there is a black powder cartridge orgainization that uses the range at Friendship every month. They shoot the 500yrd berm, and they are experts at BP cartridge, particularly trapdoors.