The pair of gears on the left of the first picture are the first forward gear and the other two are the reverse idler. The bottom gear on the reverse idler has unworn teeth - they should all look like that. The Morris Minor gearbox doesn't have synchromesh on first or reverse, which means that it crashes loudly if you try to put it into either of those gears when the speed of the input shaft and the output shaft don't match. You're supposed to come to a complete stop, press the clutch, and wait a couple of seconds before changing into first at a junction or roundabout. It is possible to double-declutch down into first but it's a bit tricky to get right.
When you crash it hard, the corners get chipped off the teeth of the gears. Do it enough times and they become tapered as you can see in the photos, which means that when you apply torque the gears are pushed apart, popping the stick out of gear. In a slightly worn box this usually happens when you're climbing a long steep hill. On a badly worn box it does it all the time unless you hold the stick in with one hand while steering with the other.
The fact that the smaller of the two first-speed gears is in much worse condition than the larger illustrates an interesting principle - they've actually worn by about the same amount but the wear is spread out over more teeth on the larger gear.
The rest of the gearbox was in remarkably good condition. All the helical gears and the synchromesh units are perfect, the main bearings seem fine, and there wasn't much wear on the layshaft. I think it must simply have been repeatedly crashed hard into first gear until the teeth were chipped away.