I don't know where you draw the line between "proto-prog" and "prog rock." I've always considered them prog rock. Although they started in the late 60s and their earlier, Pink Floyd-ish type offerings may be considered "proto-prog", I suppose, and then they did a Fairport Convention-ish "Brit-Folk Rock" album "John Barleycorn Must Die" in 1970, I really cannot find a better category than full-fledged "prog rock" to put "Low Spark of High Heeled Boys," which came out at the end of 1971. It has a lot of jazz fusion influence, but that does not exclude it from prog rock. They released a few more albums after that, but none as good as this one. Take a listen, if you want, and see what you think.