I'm not able to test it, as I don't own a ZX81, but it looks to be fine. It isn't marked, and seems to be 3rd party. Probably not homebrew judging from the PCB though.
I asked someone about it a few years back, and they said it was 16k, from memory.
Amiga 4000: Apollo Turbo 040, 128MB Fast, 40gb HDD, CD-ROM (needs internet)
C64: 1541U-II, MixSID, WiFi, LumaFix64
ZX Spec, A1200, CD32, MacPlus/SE, A500, BBC Turbo Master (NASA), 2600, Acorn 4000, VIC-20, IBM 5155, GameBoy