Hello Alec,
That question has been puzzling us for years. To the best we can surmise, it must have been an issue of reduced costs and knowing the Stag was going to be discontinued in the American market at the end of 1973, as Stag production costs were sky rocketing. The Stag increased in MSRP sale cost an average of $1000 a year from 1970 to 1973 with an entry sale price of $4995, and an exit sale price of $8995 in 1973.
Needless to say it really ticks us off that we got the crap axles (staked style with small u joint)in all our Stags, certainly in the target market that was supposed to carry the major numbers for sales!
What is really puzzling is that TR4IRS through TR6 all had circlip universal joints, much higher production numbers and a design that is the exact same as the Stag except teh spline axle. Only part number differences between the circlips and staked style, and the Stag spline axle is 2" longer than the TR models. We grab a set of TR6 axles, machine the Stag stub axle yokes for the larger joint and circlip groove and end up with a set of rebuildable axles.