Dan there's a master volume later 70's twin at emerald city guitars with those same speakers... I tried it out with a tele and they sounded very good, not quite as good as JBL's, but still nice. I saw that twin on Craig's list this morning as well. That's a good price if all is working well...
I remember having a Twin in the 1970's with Gauss speakers. I would say they are closer to an Altec Lansing speaker than a JBL. Just my opinion. They were great speakers.
I have a mid 70's fender Vibrosonic that has a single 15 orange frame with black magnet and it looks exactly like your picture other than the size.The label in the center of the magnet reads;Fender musical instruments Cerwin Vega model ps15.
Gauss and Cerwin-Vega were both competitive with JBL in that day, relatively the same efficiency, slightly different response. I know some who preferred the CV over the JBL.