If you have the screws you have the answer, not me.....
The diameter of the thread is, is the diameter of the screw. I'd expect this to be some multiple of 1/16, I.e. 3/16, 1/4 etc. I'm betting on your screws are 1/4"
Next you need to measure the pitch. Not as hard you first think.
Count either the number of threads in 1inch.
Or measure the length of 10threads.
If the screw is 1/4 diameter and UNF you'll have counted 28 threads per inch. (or 10 threads are .35in or ~9mm)
If the screw is 1/4 diameter and UNC you'll have 20 threads per inch (or 10 thread are 1/2in)
Then measure the length of the screw.
Spec the screw by dia, thread type and length. E.g. 1/4 UNF x 1in
BUT the best thing to do is pick your best nut and drop to the hardware store then return later to pick up the screws that fit. If it's been acid dipped it will be clean.
Which is probably what you normally do.
Note: UNF and UNC only go down to 1/4 dia, if the screws are less that 1/4 you'll be into BSF which have the following threads per inch
7/32 BSF dia (good luck measuring that by eye) has 28 threads per in.
1/4 BSF has 26 threads per in, so close to the UNF you can see how we all wrecked threads when young.
However, I'm sure someone here can tell you the answer to you question if you wait long enough.
G.