The rear brake switch on all the big twins since 1958 is a part # with a -51 suffix. Would the servicars have had juice brakes starting in 1951 or where else would these switches have been used prior to 1958?
Yes, Servi-Cars received rear juice brakes starting with 1951 models. In my H-D catalogs the earliest listing I have for switch #72002-51 is for 1951 S-Cs. The same PN is also in my 1961–71 H-D BT catalog.
According to page 592 of Palmer's 37–64 SE there was a change to the switch as of 1969 models. He says it was altered to the spade-tip type with the male terminals on the switch and the female terminals on the harness. However, I cannot confirm what he says.
Eric