From the Free Dictionary:
". (General Engineering) a short cylindrical projection on one component designed to fit into a hole on another, esp the male part of a joint"
In this case, the 10mm projection on the prop hub that fits into the propellor when using it to drill the holes in the prop. Similarly, the larger 25 mm that fits into the propellor hub when drilling it for the tapped holes to receive the machine screws.

I'll post a screenshot of the drawing of it later when the site is working better. Sometimes with my poor line speed, it just does not work.