[Frustrated Bassist]

Those screws have a whole bunch of uses. They could be left overs from changing a ceiling fixture if they are #6. Screws that long don't generally fall out of electronic items on their own (walking out due to vibration). They have a size, #6 and #8 being the most common and they come in coarse or fine thread for 'general purpose', those being coarse. Speaker mounting screws are a common use for these and cabinet staple nuts for speaker mounting can be #6 or #8, coarse or fine, depending on the cabinet.
Flat head screwdriver versions are also common for butterfly wall fasteners.
They are the same dia and thread pattern but not two identical screws from the same item/package. Any experienced electrician would immediately spot the two main differences between them.

 

[Frustrated Bassist]

Slotted screws create the best tones, as long as the slots are oriented the right way.

Especially on pickups...

 

[Frustrated Bassist]

Ever seen metric screws with slotted heads ?

Yes, in a number of industries that use screws and metric for machinery production.

 

[Frustrated Bassist]

Maybe if you live in the USA. I think I'd have to ask for a custom order at my hardware store to get slotted head screws. They went the way of the rotary dial phone and 8 track cassettes.
View attachment 616780

I live on the edge of Toronto, a two minute drive from the original Robertson factory and head office. The image you posted has a flathead/Robertson combo head. I have a number of butterfly packages bought in Canada that have flathead only. Most or all home lighting fixtures also use flathead screws for mounting the adjustable hanger bar to the box and the fixture to the bar in the op's size range, because they aren't made specifically for the Canadian market. 'Machine screws' are still available in flathead in Canada in any size, SAE and metric, including at Home Depot for the more common sizes.