Would these be 'old' necks? I thought Fender had largely gone over to rosewood by 1966?
Yes mainly rosewood fingerboard but Fender still produced the odd Precision with maple cap veneer neck throughout the early-mid 60's. I bet they had a very small stock of necks at hand for special orders
Here is one from '64 (now belongs to the dude from Artic Monkeys):
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And another one from '65:
http://www.frettedamericana.com/product/1965-fender-precision-bass-1
here is another one I've just finished.