This is a simple mechanism, in which anyone with a bit of methodical measurement/comparison could diagnose the cause of the reported problem, with ease.

With all due respect, several people (e.g. Guy ggggr) have made solid suggestions and/or observations, which you seem to just ignore, preferring instead to post more pictures and questions.

Perhaps you should take the gun to an airgunsmith, or see if a knowledgeable/experienced member lives near enough and would be willing to help you out.