One example is a relay tester. The stupid plastic relays. They are in theory easy to test. Apply voltage to A1 and A2. the contacts reverse, when it works right. BUT, try doing the test. Its an extremely large PITA. The good lord didnt give me enough hands to accomplish the mission. How can you hold 24 volts on A1 and A2, and use an ohm meter to see if the contacts change? Just try it one time. And making a bunch of wires up to test one, is ok, but soon the wires are lost, or used for something else.
So what I used to do is get me a relay socket. I mounted them on C ration cans, (this is revealing my age) and wire up some very small lights to it, along with two test points I scrounged. I made the normally closed contacts red, the normally open contacts amber. I used two multimeter leads with roach clips to connect it to 24 volts, via the test points. I installed the same test points on all my different test equipment. That way, all I needed to do is grab some leads, and what ever home made test equipment I needed.
I installed a small push button switch to energize A1 and A2. Plug in the relay. Hook up the leads. Red lights should come on, if the relay is good. Push the switch, the red lights go out, the amber came on. What could be simpler? Easy to make, and in today's world, LEDs are a fantastic idea. Much smaller and easy to work with.
The normal joe doesn't need something like this. He can make it, and it is a great tool, but someone who fixes gen sets to sell, shouldn't leave home without one!
I made all this stuff in C rat cans, because when you are on 24 hour duty, and don't have anything to do, you get creative! I then soldered a C rat can top, to the bottom of my tester, to keep out the dirt, or some fools finger. Later, when I was doing Patriot power generation, I mounted all this stuff on a work bench. I made a 28 volt power supply to power all my toys. It was a great way to learn about electrical stuff, and practice soldering and electro theory. I almost burnt my work bench up with the first version power supply. It was a valuable lesson. Fuses. You leave out fuses, at your own peril.
There is bound to be people out there that have a spare parts gen set. Or simply buy two sockets. Don't forget the AC volt switch, (S16?) or what ever it is. There is no end of the toys you can make.