This image is from the article "Restoring a ZX Spectrum+ Toastrack at https://celso.io/posts/2025/06/28/toastrack/
It was a membrane keyboard; the article has photos of the internals. The Spectrum's operating system was BASIC, as was common on many 8-bit machines. The printing on the D-shaped keys is for commands for the BASIC interpreter; they saved typing.