Stephen,
I have two versions of the 5/16-25 nut, newly made in heat treated chrome molly alloy steel.
#z385 is a tall nut 5/16 high, 7/16 inch across the flats.
#711 is 0.525 inch across the flats (5/16 Whitworth wrench opening), 1/4 high.
#735, a 5/16 heavy flat washer, that was 0.072 inch thick and about 0.620 inch diameter.
#z385 & #711 are $2US each in bare steel, or $2.50US nickel electroplated.
#735 is $0.25US in bare steel, or $0.50 nickel electroplated.
-Doug