Sakmyk, I concur with those that have guessed that the 2 3/4" engraving and chamber length is probably original. Although chambers were, and are, regularly lengthened on both sides of the pond, it is not necessary nor common to find it ENGRAVED on the gun. Stamped as part of the proof marks but not engraved.
The other question you ask is about the disc-set strikers.
On the Lancasters of this age, they are held in place by the pin in the fence and if this is worn, broken or otherwise a poor fit, the striker spring will push the disc proud of the breech face but it will be easy to push it flush. Just have a pair of better fitting retaining pins made up and your problem will be solved.
Another matter you should be aware of is with the ejectors, if it so equipped. In their quest to avoid paying other inventors a royalty for using their patents, they used a suite of highly individual mechanisms in their guns. You may have the ubiquitous Southgate, or Deeley if a conversion, but you may have the Perkes which looks like a Southgate in passing but when exposed, the spring looks like a snail on LSD (it performs the functions of both sear and tumbler) and is very probably irreplaceable. DO NOT BREAK OR LOSE IT!
