Most likely the 16K "rampack". It was pretty much essential to get anything done with the machine[1]. The rampack infamously suffered from "rampack wobble" where a slight movement of the machine would disconnect lines between the motherboard and the rampack causing a crash. Eventually I got a larger case/keyboard and mounted the rampack internally.

[1] Though there was 1K chess, a mostly complete chess game in 672 bytes https://en.wikipedia.org/wiki/1K_ZX_Chess