2 3/4" 16's were certainly available before 2 9/16" were discontinued. In fact, going back as far as 1940--by which time LC Smith, the holdout among American sxs makers when it came to 16ga chamber length--finally went to 2 3/4" chambers as standard. So it is possible that box of vintage 2 3/4" 16's is old enough that American ammo makers were still offering 2 9/16" shells at the same time.
Would be interesting to know when the short 16's were finally dropped. I have a Shooter's Bible from 1960, still showing 2 9/16" available--although a definite minority by then, and everything listed as a 16ga target load--from Remington, Peters, Winchester and Western--was 2 3/4".
