I finally pulled my 64's full-foam seat apart for side spring replacement, and foam assessment. Once stripped down, the foam looks surprisingly good, but is very soft and broken down on the driver's side. Like others here, it is marked 3816940.
So after some searching, the best price right now—by quite a bit—is at Classic Industries, for $182 + oversize-2 shipping. That shipping (from CA to OR) is normally $50, but if you catch one of their frequent "free ground shipping" promos they discount the shipping. Was only $26 to OR with the discount.
The kit (SF207) includes the flat foam pad for the backrest as well. I'll report back once I have it in hand.
https://www.classicindustries.com/pr...rts/sf207.html