Quoted from xsvtoys:

Just to add to this, I always enjoy learning new things. I was reading about how Sems screws are made and it sounds like a multi step process that is more involved than just making a screw. Then I looked at the pricing at boltdepot.com.
A 1” 8-32 zinc plated steel Philips head Sems screw with external tooth washer (#26804) is 23 cents each, or $13.85/100 making it just under 14 cents each.
The equivalent 1” 8-32 screw (#1555) is 8 cents each or just over 4 cents each for 100. The #8 external tooth washer (#3002) is 7 cents each or just under 3 cents for 100.
So basically the Sems screws cost about twice as much, which makes sense given the more involved manufacturing process.
A pinball restorer probably would not care much, as it will cost about $14 to get 100 of them vs $7. But if you are cheap and like to save money, you can achieve the same look and mechanical fit by buying them separately and putting them together yourself.
A manufacturer buying them in large quantities would see a bigger cost effect. But I’m sure it’s well worth it to them, because the alternative during assembly is to have the worker grab a screw then a washer then fiddle with them to put them together. This will take more time and manual labor is expensive. Also, the worker can get tired or bored and make mistakes like leaving the washer off or putting on 2 washers. This will never be an issue with the Sems screws, just grab and go and it will be right every time.
I can totally envision the pinball manufacturers making this decision to speed up and reduce the cost of construction.
Overanalysis complete.

Quoted from flynnibus:

Thr toothed washer is used to get more bite to be resistant to vibration, etc. there is no value besides convenience to using sem screws verse toothed washers.
I would be suspect of anything with sem screw with additional washer under it. That entirely defeats it’s purpose. The purpose of the teeth is to create extra friction between the bolt and the surface to resist rotation. A washer under that defeats it.
You would normally see these in pins in metal brackets like coil brackets… and sometimes used with plastics to get bite.
Just buy a box of #6, #8 star/tooth washers and call it a day.