There is a 12A built LOS Z/28 with the "short" rear stripes that was sold new in Santa Barbara where I lived and there is also this photo from the July '69 issue of Motorcade magazine that shows a '69 Z with manufacturer plates and the same short rear stripe treatment. This car was described as silver with a red deluxe interior.
It may well be that it was hit and miss as to when the short style stripes happened. I do think it may have been limited to the Van Nuys plant but I don't know that for certain. Jerry encountered one of these cars doing an inspection last year. The guy had pics of the car when it had the original paint and the short style stripes. As a Camaro owner back in the '70s in California and going to car events, I remember the occasional Z with the short stripes. I didn't know why there was a difference but I just was aware that it was an authentic GM anomaly.