I can’t really put a date on it. The primer, base style, color of the brass, etc seem to be close to the prototypes that I have of the 30-80WCF, 6mm Winchester and 358 Winchester, so I’d lean toward that period - 1950s & 1960s. Also, the Silvertip bullet has the same general “look” as bullets from those years which could be another indicator.
I have seen nothing to indicate that Winchester was contemplating new cartridges based on any cases other than the T65 and the belted magnums immediately before or after the introduction of the 308W. If they were, they kept it a well guarded secret.
On the other hand, the short & fat craze didn’t begin until the 1980s & 90s and the cartridges by Winchester and Remington weren’t fully developed until the turn of the century so it would seem odd if this cartridge was a prototype that laid dormant for 30 or 40 years.
This is all pure speculation, I know, but you have to start somewhere. I posted the photo here and in the JOURNAL hoping someone would recognize it.