This is a bit of info from guitar world , apparently the slab board was to give more bass because of the increased depth , @onehandclapping Has a very nice 62 slab jazz , maybe he could tell us more about them 🙂
With all Fender models, the early ’60s brought the “slab board” fretboard, as opposed to the “veneer board,” which was introduced in mid 1962. The slab board was a thick piece of rosewood that joined the neck on a flat plane; the veneer board was thinner and joined the neck on a curved plane. Clay fretboard dots were standard until the middle of 1964