Hello. You have a CTS loaded bass cab. The 137 in 137-7104 is the CTS factory code and the 7104 means the fourth week of 1971. CTS speakers were also used in some Fender amps too, and for a bit of trivia, i heard/think CTS stands for Central Telephone Service; go figure. They sound pretty good for guitar too if you crank the treble on the amp you are using. Kustom did make 3-15 guitar cabs as well as PA and bass cabs, so it was a fairly widespread format. Good luck!