Modern Vespa

For years I have been intrigued by the idea that you can replace one gear in a set with one with a different different tooth count. I can't get my head around this.
Recently I came across a picture of three primary gears : 19,20,and 21, that would mesh with a 64 tooth ( IIRC) clutch gear. I can't find that pic. Anyone know where I saw it ? I'm thinking it might have been here ?

Was it in Rolands Project Snowball posts?

1984 PK 50 S - the Snowball.

Yes ! That's it. Thanks. I still don't see how this work without some pretty severe wear at the least.

Be aware that when you change only one gear it changes the Pressure Angle (the line along which the force is transmitted). This shift in the direction of the force can impact the force on the teeth as well as the force of the case supporting the gear.

It can be done, but it's not as safe as replacing both gears as a matched set.

Exactly. I have no plans to do this, myself, but have been intrigued by the fact that the ratio can be changed by altering the number of teeth on one gear. But there's the picture and it apparently, it works.
It still seems intuitively wrong to me. Having replaced teeth on wooden clock gears, I know the contact points, tooth angles, and mesh surfaces are critical.