Our '68 GTS convertible carries an 8/28/67 SPD. According to the original owner's manual it was first sold in May 1969 from Northwestern Dodge in Detroit (Ferndale). It was also sold again in early 1970 in San Francisco with about 13,000 miles. That's a lot of miles in such a short time in those days. I suspect without any proof that it was used as a demo even though it has so few options. It was also, as far as I understand the codes, a sales bank car.