Does anyone know what this part from the S88 MKII is and where it belongs?

Probably not very helpful but that sure looks like a standoff / spacer, usually used to support something that lays on top of it or to just separate two parts.

I'm guessing you disassembled your keyboard, in such case just look for a place with a screw hole or bare screw that is not connecting to anything and fit it there to see if the pin part makes contact with another part for support or separation.