Slight repost, here's a pic of tools used in cutting and measuring threads on fasteners.
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If you have the die for the threads you want to cut, you could buy a kit from Brownell's gun smith supply and make your own screws, scroll down on this link for the kit and refills for flat head screws,
https://www.brownells.com/UserDocs/M...itsSection.pdf
Now I won't go into the different kinds of flathead screws there are, but let's just say, they can have different depth of head for thinner materials. And some have "undercut" heads.