I think the major difference is that many of the early instruments were oil finished. And after that, we used a polyurethane finish that was later discontinued by the manufacturer.
Also, there's a huge variety of woods in the both the bodies and the necks in the 70s while we were learning what sounded good. Not every experiment produced a buttery sound. I think if you're after a predictably buttery sound on a newly built bass, then straying from our current typical neck arrangement of Maple and Purpleheart may be advisable (think Brown Bass).
The electronics and pickups are only changed as certain components were discontinued (like the original ICs that were discontinued only months after my dad designed the original PF circuit). Many of the components, like the pots, are identical.
There are some older basses that don't have filters, like the P1,P2,P3 and P4 series. Ask dela, these sound remarkably different. PF5 had filters, PF6 was the last major revision, we're on PF6B now, with only minor changes to reflect the changing environment of high frequency noise sources.
Oh yeah, another big difference is that they are old. On an older instrument, what has been vibrated through playing, the cell walls thin out and make the wood more resonant. Just sitting around won't do this, the bass must be played to enjoy this benefit.
And to the discerning ear, no two Alembics will sound exactly the same. I think that's what we love about them, they are individuals.
