You might try rubbing just a bit of transmission stop leak on that breech seal and let it stand overnight. I've used it to revitalize flattened o-rings and breech seals with good results. Won't fix the problem but might swell up the seal enough to keep you shooting until you can track down a replacement.
... Not one bit sure what the thin metal lever is for but if someone figures that out, by all means add to the thread....
Sergio-ATF (automatic transmission fluid) stop leak is an inexpensive product that softens and conditions gasket material. Any auto parts store will carry it. It works on o-rings and synthetic gaskets. It is often a miracle cure for leaking CO2 guns or older CO2 and pump guns when the seals begin to harden and become inflexible. You do NOT want to use it on PCP airguns however, as the pressures they operate at are much higher and may cause an ignition or explosion inside the air resevoir. For your particular breech seal application it is worth a shot. Crack the barrel (but don't cock it) and dab a thin coat on the breech seal. Arrange the gun so the barrel won't close and compress the seal, or stick something between the breech and the barrel (shim, piece of plastic, etc.). Stand it on its barrel overnight. Wipe off any excess oil when you are ready to try it out, and give it a go.