From Whotabs.
From November 1975, Guitar Player interview
Once I realized the Thunderbird was the bass I was going to be playing for a few years, I panicked because they stopped making them. I went to Manny’s [156 West 48th St., New York, NY 10036], and told them to buy up the whole stock, so consequently I got ten two-pickup Thunderbirds. Some of them had harmonic, microphonic pickups, so I couldn’t use them, and with the rest, I just couldn’t get the action as low as I wanted. So I used the pickups and hardware and bought some blonde Fender Precision necks and had some old style Thunderbird bodies built for the Fender necks to fit, and I’ve been using them on stage more than the Thunderbirds. I use them for recording as well. Blonde Precision necks are my favorites; those and Rickenbackers. Thunderbird necks tend to be too thin down at the bottom. I get my fingers all tangled up when I go to the first fret.
I guess I was wrong here...