This is an old post, preserved for reference.
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When the ZX80 was released in 1980, it came with a version of BASIC in a 4K ROM chip, known as 4K BASIC. Note this has nothing to do with RAM (the ZX80 has 1K, upgradeable to 16K), it is the size of the ROM chip.
These only come in the version with the grey keys and 8K BASIC keywords.
My thanks once again to Grant Searle, these were adapted from his ZX81 overlay artwork.
I have revised the design slightly, moving the top row of keywords above the buttons, as on the ZX80.
2022 Update: At the time of writing, there are still a few Minstrel 2 (ZX80) and Minstrel 3 (ZX81) kits available from The Future Was 8 bit but with keyboards rather than overlays. Those are the Final Edition kits, there are unlikely to be any more.