Not completely true. From 1966 onwards the "maple cap"
neck was available at custom order (and at an extra cost). You can see those "maple cap" necks more often on the guitars (The black telecaster Dylan used in the 1966 tour with The Hawks/The band, The Hendrix Woodstockcaster,...), but on bass they are not that commonly found. I know Roger Glover
had a maple cap P-bass, but that's just it (apart from Entwistles slab-basses)
I saw one 1968 Jazz Bass with a maple cap, but that was a long time ago, so I guess there weren't many made, since it was truly a custom-order. As for the blocking. Since the maple cap was only available from the 2nd half of 1966, they were always blocked, since the blocks & binding were also introduced in the summer of 1966. This is only true for the "deluxe models" (Jazz Bass, Bass VI, Bass V, Jazzmaster, Jaguar). Ordinary models (P-bass, Telecaster, Stratocaster) just had dot inlays with an unbound neck.