When it comes to splined RS bosses there are only 2 sizes of spline but they differ in depth, Mk2 escort fiesta and cortina are the same (and early Mk2 granny), Mk1 escort, transit and capri are the same.
I have a fiesta boss on my cortina cos i couldn't find a cortina boss
Unfortunatly Mk2 escort ones are the easyest to find but the most expensive mind you they are very deep and will push the wheel towards you about 3 inches



Might be easyer to make an adapter ring and use some other boss, thats what i did till i got the fiesta one ?

You can see how shallow the fiesta one is on a cortina.