I assume this is for the 1969 GT-500 clone referenced in your profile. The Ford hardware part number shown in the 1969 Mustang Electrical Assembly Manual (AM0028, pg03), 393141-S is incorrect. Note that it is also incomplete, no finish.
The correct number "could" be 383141-S, 8-32X3/8 PAN 5/16, XREC, THD FORM, which is used in 1971 and 72.
"Ford Car Parts" (aka MPC) shows 375452-S100, 8-18X1/2 PAN 5/16, XREC, BLUNT, SST, as the service replacement (also common for years 65 thru 69). Typically, a stainless steel screw does not require a finish, but it could be further "treated" to resist weather.
I hope that does not confuse you.
Jim