This is an adapter of the early 2000’s: the Griffin iMate. When Apple finally decided to switch from Apple Desktop Bus aka ADB to USB for input devices, many users were confronted with having incompatible keyboards – the iMate could resolve this issue. (Btw: I love proprietary interfaces … opportunity for many adapters to be produced).
After all – it’s nicely coloured and perfectly “fits” the design of the blue-and-white PowerMac G3 as well as the G3 iMacs … erm …
Part one of the “Magic adapter box” (see post below).