I've seen them on Pierces and Lincolns, among others that I cannot remember right now. The ones I remember were Autolites. Ford's were round, both on A's and V8s.
ive got a couple of square cutouts i recieved in some parts. if the ice thaws to wgere i can finally get my shop door open i will see what brand they are and my AA is running one now.
I have one on my smiths compressor ---it has enough mounting holes and slots to mount on anything, it came out of some WWII surplus ,--I used it because it was new, and free.
The generator/starter guy I use keeps them in stock.They are kind of a universal thing,rectangular with a bunch of mounting holes.A lot of old tractors use them.I have a couple that were sold by Western Auto.They are pretty substantial,big copper coil.