First thread. Old school body work question.

Morning all. As I am currently undertaking my first major restoration/modification of a classic vehicle, I am kind of figuring things out as I go along. I am aware of old bodywork techniques such as using lead for filler before the birth of Bondo, but when Bondo showed up, it seems that it was the weapon of choice, so to speak. Now, here is my question. The 1958 Cadillac I am working on seems to have had a rebuild done on it in the past. I found a plate riveted to the firewall from a place called Garage Supply LTD with a date of 8/16/63 stamped on it, and a job number of K7072. From the Google, the only logical place I can find for this place is in Orillia Ontario, Canada. Does anyone know anything about this company? Were they doing work like this back in the '60s? Nearly the entire car has a skim coat of Bondo covering it, in some places 1/2 inch thick, yet the metal underneath seems solid for the most part. Was this common practice back in the day? Just trying to do some research on my project as I go along. Sometimes you have to know where they have been before you know where they are going. Thanks all.