In Europe most 626s are double triggers, so there should be parts for the conversion at some dealers or Beretta itself.
Seeing that the system is not resetting itself, yet it can be manually changed to the second barrel, it seems that something is blocking the inertia block from moving back under recoil. Check if you can freely move the inertia block back. A worn pin, or some debris trapped somewhere under the block might be the problem.
The inertia system is so simple that it cannot be anything major.
Thanks for the really great info that a DT 626 does indeed exist. You have just given me a quest. The block will freely move back, but it doesn't do anything that I can see to pick up the other barrel. I got a return call from my local gunsmith this morning and carried the gun to him. He said he would do his best to fix it before my hunt, but parts might keep him from being able to get it done by then.
