My 50 wire diagram show no relay 6v
http://chevy.oldcarmanualproject.com/electrical/wiring/images/50ctsm1203.jpg

Cannot find a 52 diagram

Im 12 volts and added one, it was a 12 volt relay.

Im not sure if relays are voltage sensitive.

53 truck wire diagram shows a horn relay 6v

http://chevy.oldcarmanualproject.com/electrical/wiring/pdf/53truck.pdf


So your right , at some point they were eliminated then re introduced