Hard to get a clear picture but this shows the F
The 2 on the left are from a 28 spline axle and the right one is from the 31 spline.
The 31 spline version has the F much more pronounced and a bit larger. The heads are all the same either 28 or 31 spline.
I'll get a set off the shafts today.
Richard
PS: I'd never thought about bolt head/axle stud markings being specific for a Concourse restoration, but then I found that there was a need for a specific nosecone for an original Cobra starter and I happened to have one but had to check at least 15 starters. Can't remember who needed it. Or the 1967 427 Fairlane thermostat neck that I got off a F427 I'd bought. Go figure!