One millimeter is 1,000,000 nanometers. So that's roughly the 1950s. :)

maybe around 1956 or so

I think it's a question of mm rather than nm!

https://en.wikipedia.org/wiki/Bendix_G-15 was much slower.

And had much larger "nodes", given that those tubes are much, much larger and farther apart than the SMT transistors on this 6502 replica. It's not even close.

But I think IBM's SLT from the 1960s might come close: https://en.wikipedia.org/wiki/Solid_Logic_Technology

Those little square "chips" are not actually integrated circuits, but effectively small PCBs with surface mounted discrete components (including transistors).

The early PDPs used discrete CPUs, so they'd probably be the closest examples, the 12-bit PDP-5 is the smallest those CPUs went though, which isn't hideously more complex than a 8-bit CPU.

I thought about CPUs with either discrete transistors, or with e.g. 74xx ICs. But the former intuitively seemed too big to me (because I was assuming they use large transistor packages instead of SMT), and the latter too small (because a single 74xx logic chip can pack a lot of transistors).

So this “hybrid” of having SMT components in a can by IBM seemed close.

Happy to learn more!

Before my time but isn't 1956 too early for Silicon transistors?

I'd have put it around 1963. Before DTL chips, introduced in 1964. Non-military TTL came along in 1966.