Can someone tell me what this part is? I wanted to make something from the &quot;Forbidden LEGO&quot; book and I came across a piece, I don't know what the part number is.

The piece has 5 peg holes, one axle hole, half a gear, last of all I don't know, but in the instructions, it looks like there are 2 half peg holes. What is the part?
This is Technic Gearbox Half. It's very easy to find by searching &quot;technic gear&quot; on BL.

The item number is 32166, and it was available in the early 2000s (specifically from 1999-2002).