The two types of machine screws we've seen directly from an original Wah Chang communicator (in Alpha) are: 1) 1/2" brass 2-56 slotted oval head, and 2) 1/2" steel 2-56 slotted round head.  The "2" in the 2-56 number is the national standard (ANSI) reference diameter and the "56" is the number of threads per inch:

These two types, however, are not the only ones used in the comms.  All told, there were likely four:

In chart form, they sort out like this (as best as screencaps show):

Now then, if you were to choose but one type to represent the quintessential screw for your replica builds, that would easily have to be the 2-56 brass slotted oval head.  That's becaues the overwhelming majority of screws we do see clearly (on Beta, Gamma and especially Kappa) are the brass oval head:

The above diagram is from measurements of the one on Alpha.

Where to get:
You'd think that finding this 2-56 machine brass slotted oval head screw would be easy.  Unfortunately it isn't made anywhere in the world!  That's right, the very one we seek the most hasn't been in production for years.

Your options are limited:

      1)  Search for someone selling an extra stash on eBay.
      2)  Use instead the brass or steel slotted round head, obtainable in small quantities from Micro Fasteners, Fastener Mart or Amazon.
      3)  Take a brass round or pan head screw obtained from eBay or Amazon and grind / file / sand the head to make it more oval:

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