That model was fitted with a mechanical 5 speed constant mesh gear box.
To change gears, you have to stop dead then shift , a method of drive which seems to be beyond the understanding of the highly educated people now days
When you try to shift while it is moving you rip the dogs of the shifting keys and if you are really lucky smash a few gears as well.
The box has 5 gears on one shaft that are fixed and always engaged with another 5 gears on the other shaft like the image below
View attachment 37507
This is called a constant mesh box because all of the gears are always meshed with each others.
The lay shaft has 2 groves running full length and in the groves are sliding keys that lock one gear to the shaft to make the mower drive as shown below
The long thin things are the keys
View attachment 37508
These keys have broken off so the 5 gears just free wheel on the shaft and the mower goes nowhere.
The brake works on the layshaft so while it will be locked solid, because the gears on the layshaft are all spinning freely on the shaft there is nothing to stop the main shaft from turning.
View attachment 37506
As easy fix but very messy as you have to split the oil ( or grease ) filled tranny