My 67 Jazz that I bought in 68 had a “tug bar” too, and I started off playing with my thumb too. But none of my bass heroes at the time were playing with the thumb, nor any of the few electric players I knew in my area, so I quickly learned alternating technique.
I sometimes revert to thumb playing even now, and like the result too.
George Porter Jr plays with his thumb, or used to. When Monk Montgomery picked up the electric, he used thumb technique like his brother on guitar. I think he switched to pick later.
I’m really kind of surprised that more players don’t play thumb technique. A nice fat sound is possible, and wrist angle is better too. Of course many bass players use it when palm muting.