The axles in your picture are the "second" version I mentioned. The small groove next to the splines was added in an effort to allow the axle to "twist" a little there and not break. Car Craft magazine ran an article on this axle style in '66 . I don'r remember the month but the cover was yellow with a red Chevy II ( Dick Harrell IIRC) doing a wheelie . The article read like "axles with a twist" or "new twist on axles" . It details the undercut these axles have and the theory behind them. As I mentioned Holman Moody sold them over the counter "in the day". IIRC ten '65 Mustangs were converted by Holman Moody and one was built by Dearborn Steel Tubing. A few cars still exist and some have been crashed and are lost. IMHO the axles have about the same value as the '67-70 31 spline Mustang axles . As you found out they are not well known. The axle housing does look like a 57-59 full size housing and the 31 spline axle would work in those housings unless it was a wagon or Ranchero which used the larger axle bearings. That housing was a popular swap "in the day" as the "K" code housings were hard to find and expensive at the dealer.
Several aftermarket manufacturers offer "modern" new 31 spline axles in the $300+ range. Many guys don't feel safe with a 53-54 year old axle in their car and buy the modern ones.
'66 GT350H-3 time cover car - Car Craft July'77,Modified Mustangs and Fords Feb2011 ( w/article), Mustang Monthly June 2014. Bracket raced by me for the last 44 years. Yes it is a real one.
'68.5 R code GT fastback
'67 Ranchero 408W -427 glass hood
'78 F350 "Oleynik" car hauler
Ford Experimental parts collector
Last edited by GT350HR; 08-29-2019 at 11:21 AM.