Thanks Len, that's very interesting! I've never seen that second style of blade. FWIW, I have another Mk 3 with a 19,xxx serial, and its trigger blade is identical to my 16,xxx one above.
It would be interesting to know if that was a permanent change datable to a particular serial number, or some sort of temporary aberration along the way! These sorts of changes in old airguns are always fascinating for sure.
These things always remind me of a humorous article that was in "Gun Digest" years ago. It was called "The World's Greatest Webley Collector," and was about a guy who had gone insane in his attempt to own every Webley revolver variation!