'Ron' - Some simple questions have difficult answers.

IMHO - With U.S. Ordnance screw sizes - Fantasy becomes reality! It seems things kind of evolved and were maybe tied to British and French military weapon screws of the 18th century??? This is an area ripe for research by someone more competent than me, with a knowledge of machine tools.

However, I did measure a 'slide lock screw' for a 1902 Krag sight. The maximum thread diameter was .171 inches. Using a thread gauge, there were approximately 40 threads per inch, (that was the finest thread allowed on my gauge set).

Counting threads and measuring, my calculations arrived at about 43 threads per inch.

My best guess is .171" X 43 t.p.i.

FWIW - The 1902 Frankford Arsenal tap & die set information (posted by 5MadFarmers, years ago) had three taps that used 43 tpi, but, nothing that large in diameter.

A tap for cleaning the threads would have to be custom made. Adding to what Butlersrangers already said determining the correct size is very difficult as the size and pitch of these old  screws changes from one end to the other. The tooling of the day was very imperfect and run till it could no longer be resharpened.
If you are not dealing with physical damage (just gunk or rust) to the threads perhaps you could use a 17 or 20 caliber brass brush to clean the threads. Use along with a good solvent. A 20 caliber brush would probably be ruined doing this but may be worth it.
If you are dealing with mechanical peening of the threads, a machinists burr knife could be used to remove the offending area.