Old style bolts with a slight dome. Machined from 4142 steel that is heat treated equivalent to Grade 5. The correct style for many vintage vehicles. 

Black Oxide finish

size - 1/4-20x3/4     A) 3/4"    B) 5/8"  C) .270  D) 7/16" -  NUT307

size - 1/4-20x1         A) 1"       B) 3/4"  C) .270  D) 7/16"  - NUT308

size - 1/4-20x1-1/4  A)1-1/4"  B) 1"     C) .270  D) 7/16  -  NUT763

size 1/4-20x1-1/2     A) 1-1/2  B) 7/8"  C) .270  D) 7/16"  - NUT309