This footswitch is for the 1980-1981
Fender 30 amp, which isn't Rivera-era, but here you are anyway.
It probably won't work with any other amps.
I reverse-engineered this schematic from the amp schematic, and Nick Serra, who owns a Fender 30, had it built and it works. Just saying.
Here's the inside of Nick Serra's footswitch... he had it built
into a Fender dome-style casing.
photo- Nick Serra
The above advice is offered in good faith but I accept no responsibility for loss, damage, injury, death, blown up amps, etc etc. I'd appreciate it if you'd treat my write-up and sketches as copyright 2016 Andrew Waugh. Thanks.
Valve (Tube) amps page