Thanks @
unionrdr! A quick google search for "Cooper's fermenter" reveals you are totally right! That solves my mystery! The guy I got mine from told me he hadn't ever actually used it as a fermenter, but it was his Star San bucket for years. It needed a little cleaning (nothing a PBW soak didn't solve), and I didn't like the plastic spigot, so I replaced mine with a 1/2" SS ball valve, and it's now one of the rock stars in my setup. I actually think I like it slightly better than my 30L Speidel due to the clear part in the lid so you can easily check in on things.
The reason I thought it might have been an early model Speidel was mainly due to those little ridges in the lid which both have and the manufacturer's markings on the bottom of both are nearly identical. I wonder if maybe Speidel did make these for Cooper's or perhaps they both just contract with the same vendor to have them made. Either way, it's a solid fermenter (for me at least).