Help identify this mystery plug on three spd

[1970-Old_Betsy]

Not sure what this plug connection is for on my 1970 C/10 with three on the tree. First photo shows transmission , next is the connector, the wiring harness and then the relay. I think harness was for maybe for air conditioner blower. The only connection that was plugged in was a keyed hot wire to the relay on the firewall and the connection on the transmission. I have truck all back together after restoring it and everything seems to be working fine, even the back-up light ( switch on column ) Anyways...........what is the plug on tranny??

Thanks!