Heater Box Pal nut coating

[MyRed67]

   Can someone tell me what was the correct coating on the Heater Box Pal nuts?   I have cleaned mine up pretty good, without used chemicals or sandblasting, and they look to have been Black Phosphate.   All replacements I have seen look to be Nickel or Silver Cad. plated.   Thanks for any help.

[JohnZ]

The same 9845 nut was used from '67-'69, and they're black phosphate.