There are two styles of aluminum plates. The first was used to make horseshoe pickups. They are rectangular with rounded extensions on each end for the height adjusters. The second type was made for the early high gains and has tapered ends. The horseshoe type also have extra holes to accept the screws that secure the magnets.
My '69 p.u. looked just like the above pic, and had the little holes. And the plate was just like the pic Adrew just put up. The ends were definitely hand cut with a hacksaw or bandsaw. Very quaint! Based on the many 4001s I've owned, serviced or inspected, my thought is they all had aluminum base plates until the early 70s. Then black plastic from then on. I'd be really surprised to see a black plastic one on anything earlier than the early 70s, unless it was a more recent replacement.
the bottom 2 pics show an original early 70s high gain with the screw tops ... but I did spray paint the pickup to make it look better ... I bought that one off of ebay ... it spent some time in my old 68 then I sold it to a forum member ... can't remember who got it ... I can't remember if it had the alum base or the plastic base ... it was a good sounding pickup ... as far as I know it wasn't redone other than a coat of new paint ...