While I'm pretty sure Frontier operated its 727s into the early 1970s (I think the last of them was sold to Braniff iin 1972) I'm not sure of the timeline on Petroleum Club services. The 727-100s arrived in 1966 and the -200s came online in 1968. So far as I know, the 727s were always operated in a two class configuration, so that might suggest the PC service extended to the 72S flights as well. By the time I started working at Frontier, we were strictly standard class 737-200s, CVR-580s and a couple of DH6s.