Ray - I have pretty decent Winchester files, but after one hour of searching, I could come up with very little other than “1955” for the introduction of the cartridge.
One thing I can offer is that the barrel caliber-marking die for the winchester Model 70 in .243 winchester caliber was die number 33933-1395, and that it was authorized on February 17, 1955.
another is a factory drawing with a date of October 6, 1954. That drawing has additions up to October 1961. It is interesting that there is an entry dated May 15, 1955 on the drawing that indicates “243 WIN. was 6 MM WIN” with no other explanation. The only thing puzzling about that entry, the first revision to the 1954 drawing, is that it came so late. It would seem to indicate, at least to me, that the cartridge was actually offered to the public after May 15, 1955, but that doesn’t wash well with the normal publishing dates of catalogs in that era, which were around the first part of the year. The Winchester 1955 catalog does show the Model 70 and Model 88 rifles, the latter introduced in 1955 as well, in .243 caliber.
I was disappointed that SAAMI has no such nformation. They don’t even date their drawings, at least in the published form. I have the SAAMI Drawings.
I cannot come up with any specific date in 1955 other than this. Sorry to give such scant information. I have to go for my haircut appointment, but if I have any time when I get back, I will look a little more. I think, though, that I have hit any source I have likely to have this info. Aside from factory material, I had a couple of dozen articles on this caliber - all just give the 1955 date - and four articles on “Dates of Introduction” of cartridges, none of which give more than the year - again, 1955 for the .243. It is obvious that to be authorizing the manufacture of a barrel-marking die in mid-February of 1955, that development of this cartridge had to start in 1954 at least. As you know, these things don’t happen over-night in factories. ots of R&D, Market survey, tool and die making for cartridge production, test firing, establishment of proof loads, etc to be done before a cartridge/rifle combination is introduced.
Reference: “The Rifleman’s Rifle,” by Roger C. Rule, page 112
"Winchester Chamber Drawings & Cartridge Drawings 1928- 1962, Gig Publications.