Very nice, Lance! 50' was the earliest I've seen but I have to say that I hadn't considered even looking purposely for anything earlier. Mr. Skennertonicon mentions March 31, 1949 as the official introduction for the 1/3 and 1/2, which is the same date for the Mk.2 introduction.

I don't recall seeing any 1/2, 1/3 rifles with a "month" associated with the upgrade date and if that is the case, it will be impossible to know how early in 49' they were being done.