Pipeline, I'd say serial numbers in the 22,000 range date them to 1960. KLH used a consecutive serial number system, starting with 00001, then going up- 00002,00003, etc. My model six's are serial numbers 000711 & 000688 - making them one of the first pair made, about March 1958. They also have cabinets made of 12-ply plywood which KLH used for a very short time in 1958....and have a super retro blond korina finish. I found these at a flea market in mint condition for $20. The model Six stayed in production until 1972 and was really one of KLH's most influential loudspeakers in the acoustic suspension design era. Kloss himself said of the Six, "It's the most right sounding speaker we made".
I've spent a lot of time logging serial numbers on KLH products along with production changes and have written half a dozen articles on KLH products for various radio club newsletters. .....love KLH !