As said, the spline is 9/16 x 36 spline, SAE standard.

www.minikingdomonline.com.au (John Smidt) actually still has NOS unmachined forgings here in Oz. I looked at getting them CNC machined locally, but the cost of machining and getting a spline broach made knocked the idea over. Ditto CNC machining from bar then wire cutting the spline, the total cost made it uneconomic.
I suggest buy the Minisport.com.au ones if you have a need.

I've identified a u-joint from Flaming River available from Summit racing.... this thread has a good bit of info, and I show photos and part numbers for the u-joint.  I use these to build my custom tilt steering column you see in my Mini:

//www.minimania.com/msgThread/112544/1/1/Needed_Steering_column_part-_anybody_parting_a_mk2_or_later

Dave, Would you weld one side of the UJ to the end of the shaft, or drill the shaft and bolt it on? 

The Mk2 escort is the same spline as the mini at 9/16" x 36t

I recall hearing that you can grind / turn off the weld and slide the old female splined part out (shaft is splined under the OD female sleeve), cut a small portion off the end of the shaft, and then use the escort UJ. Even if you aren't using it as a UJ, it's cheap as chips and allows easy adjustment of the column without touching the rack.

www.ebay.co.uk/itm/141796261849

www.ebay.co.uk/itm/151809663240

Perhaps it was the price that scared me off. It would be worthwhile to grab a few anyway. I should get some pistons while shopping over there. Thanks Steve (CTR)

PS if anyone has a bad shaft please contact me. I'm up to 5 rod change shifters to rebuild on hand now. I like doing a pile at the same time. May start offering just the housing bushed and line reamed exchange. 

Minisport Australia

//minisport.com.au/mini-steering-column-spline-end-all-columns

NLA in the UK and USA Steve but apparently available at Minisport Aus part number 21A3099 not cheap at $110.00 Au/$78.00 USD.

Hey Mal, old age being what it is I thought those were NLA. I used my last spare shaft to repair this guys car. If I can find a coupling with the correct spline I can make it from there. Steve (CTR)

Kevin DR Mini has posted them here before Steve available as repair ends in Australia but i don't remember the vendor.

Recently test drove a customer car and the steering shaft splines stripped while making a U turn. I noticed a lot of play as I started out and even mentioned to the owner that we should return to the shop as I felt we had a serious issue. The rack a later type with the groove for the bolt rather than a notch was fine. The bolt was a 10/24 and not doing much. It seems the rack spline is 9/16 X 36. I can find 3/4 X 36 and 9/16 X 26. I recall Jemal doing a conversion, I'd like to be able to repair damaged shafts. Any ideas for a source, I think some Innos have a U/J at the rack. Steve (CTR)