Stationary gear parts
The stationary gear makes up the rear half of the syncro shift mechanism. When the syncro engages this gear, it locks the mainshaft/sun gear, forcing the spider gears to "orbit" w/in the planetary assembly, providing the gear reduction.
This is not a press fit, but it's tight, and may need to be tapped in w/a soft mallet.
If there's any burrs/nicks on the gear teeth, they'll bind in the aluminum housing. File them down first