Lol, a bridge rectifer. It's a small cheap universal electronic part that changes AC power to DC. Under $5 to you I'd estimate. (about 3/4"sq....or smaller) Basically 4 diodes in 1 package.
Proportionally, parts aren't as cheap as they used to be when millions were on the road. Designed for most repairs to be done by one man alone with simple tools. Strict mantainance scheduals was the biggest obstacle to even greater sales imo.
A lot like a model train, lots of fresh oil often and a peak at the brushes (or rocker arms) every other time you oil it and it will go go go 
