Thanks Brian that was great info I enjoy reading things that I am unfamiliar with great answer and thank you :beers:The original flashers were either SD - Standard Duty or HD - Heavy Duty and are available at any parts houses.
The original SD series were thermal, the HD were electronic flash and many had "bulb out" indicator technology which caused the flash to be rapid. Many of the SD series available just didn't flash if a bulb was out.
The Heavy Duty aka towing version was part number 552 the Standard Duty was 224. They came in square, rectangle or round shapes, plastic or metal and were whatever was delivered on the line that day.
Many folks today have used LED replacement lights and some of those require no load flashers that have their own grounds. There are also many more 552 version available due to their ability to outlast, warn of dead bulbs and in some cases don't care that the bulb is LED based.
More than you wanted to know I'm sure but there it is.