Hello.
That speaker is a copy of a Peavey.
Yes, a copy. A dead on copy of a Peavey.
Even down to the Aluminum side strips.
ESR made their living by ripping other companys off. Primarily FENDER and PEAVEY.
They made amps that are dead on copys of Peaveys, and also amps that are bassman and other fender circuits in ESR boxes, then they have tweed glued on, and are called, TWEED amps. No wonder this company went under.
Just take a look at this. It matches a PEAVEY in every way except for the name.
http://cgi.ebay.com/EARTH-PEAVEY-STA...#ht_500wt_1182
Rather explains the "Research" in "Earth Sound Research", eh?
I'd look into old Peavey cabinets from this time, and if you can find an exact aesthetic match, then get whatever was in that.
Also, how are you liking your SUNN Coliseum 300?
Not too much is known about ESR, except they were a New York Company who ripped others off. Been dead since before the 1980's.