To zinc phosphate or manganese phosphate:
From the June 1970 Ford Fastener Engineering Handbook.
"S2 Phosphate and Oil. This finish is a phosphate conversion coating plus an oil, black or dark gray in color, and is intended for general use on steel parts which do not require long term attractive appearance. Parts shall be evenly coated with oil. Parts supplied shall withstand 72-hour salt spray test as defined in the general requirements of section I, paragraph D-3.
The phosphate coating shall be fine or medium grain size with a minimum coating weight of 1000 mg. per sq. ft. It shall ordinarily be of zinc phosphate type, but manganese phosphate type may be used provided all other requirements are met. The specific phosphate process used is optional with the supplier, but it is recommended that all special cleaning and post-phosphate solutions used be obtained from the same source as the phosphate bath. (This does not necessarily apply to the oil used.) Controls shall be provided for solution composition and concentration, and for time and temperature of the process steps, so as to assure a uniform coating which meets the above requirements.
To avoid a phosphate coating of coarse grain size or excessive weight, use of strong acids or alkalies for cleaning should be minimized. If the condition of parts prior to treatment require such cleaning, a neutralizing rinse should be used prior to phosphating."
Phosphating has the most detailed specifications of any of the fastener finishes.
Enjoy
Scott