What is the proc rate of mystery parts?

The Pocket Planes Wiki states that:

Sometimes, during global events, there will be mystery box jobs. Once you deliver the job, you will get a part of the plane that is being given as a prize for the current global event.

So the chance of you getting mystery parts is totally random, and it can't be influenced by anything other than during a global event.

I asked some time ago When are mystery parts available for world events? if it were ever the case that mystery parts were available during events where the reward was a plane part, rather than a complete plane. There were no responses but in the meantime I've never seen it happen and I've been keeping track.

That said, my experience is that when the event reward is a complete plane, then there are some mystery parts. A reasonable way of getting them is to keep various cargo planes waiting in multiple large airports (upgraded big cities with 60 stopovers have the most jobs) and just look for the mystery parts each job refresh cycle. As the mystery parts appear (they aren't common) load them up and take them at least one airport towards the destination and then resume looking for additional mystery parts.

Update: after a few months, my experience is holding true. I've only seen mystery parts when the reward is a complete plane.