All I can add is that the plastic saddle goes in like Flighter's 'dirty' photo shows, & the lever goes in with the notch to the drivers side. 

When the shifter is installed can it be lifted when pulled straight up? 
There is a spring on the shifter under the rubber boot that may be broken or dislodged allowing the shaft to sit too low down, essentially where it would be when depressed when selecting reverse.

Put up a photo of your gear lever.

That's definitely a short throw RS2000 shifter and I just realized you posted pictures of the shifter & gearbox earlier on. Sorry for not checking. All looks good with the stick.
 
Someone pointed out previously that there should be a spring against which you push down to select reverse. I've seen a couple with springs, but most use the rubber boot "spring" which is what you have. It's the rubber boot between the with wide chrome base of the stick and the plastic cup the boot sits in as you move down the stick.
 
Holding the plastic thread you should be able to push it against the boot in an upward direction and the rubber boot will compress as the whole plastic section moves up. If you can compress the rubber boot "spring" as I've described, and I see no reason why you shouldn't be able to, then the stick's working fine.
 
As mentioned by others the notch in the stick faces the drivers side, so if you install the stick into the gearbox with the notch to the driver's side and screw the thread down all the way and you're able to select reverse without pushing down on the stick then the fork of your gear stick is sitting too low and the only thing I can think of that would cause this is your adapter spacer, which I see in the other photo you posted earlier, is mismatched to your stick, though I've never seen that happen before.
 
Have a look at this gear stick. 
 
https://www.bearingkits.co.uk/photos/1.685162fordtypeegearboxtypengetriebe.jpg
 
It is not a short throw. Look at your stick and you'll see that the plastic threaded section is higher (further away from the fork) that the one in the link. This spacer in your stick raises the pivot point of the stick, thus making it a short throw. The height of this spacer should be the same as the height of the exposed part of your spacer in your gearbox.
 
Assuming the exposed height of the gearbox adapter is the same as the spacer in the gear stick, and you can compress the stick as described then it should just work. If not, then something very strange is definitely going on.
 
Let me know how you go.
 
Chuff

Paul pull the ball and socket assembly down the stick away from the chrome section, there is a slot in the shifter shaft that an "e" clip will sits to hold the rubber "spring" down. Your clip is missing and the whole lot has slipped up the shaft.

Hi Chuff and blue74,

I can pull the nylon ball connection upwards but it is very stiff. I guess being old ( the car and me ) doesn’t help.

I have taken what I think would be all of the key measurements and marked them on the picture attached. My shaft still looks like it is sticking through the threaded section a lot mor than all of the pictures I can find on line.

Have a look up the underside between the shift stick and the nylon thread.
There should be a nylon ball fitting around the shift stick in there which acts as the pivot for the gear shift.
Due to aging the balls often have a tendency to disintegrate into small peaces and cause shift selection issues.
In your case the ball may be partly disintegrated.
The best way of finding out is to look in the grease in the bottom of the gearbox extension housing where the shifter sits under the plastic saddle, there will be broken up white hard plastic pieces.
 

Try pulling the rubber boot (ie the "spring") from the wider diameter and curl/roll it upwards toward the chrome shifter so the boot appears to be inside out.
Will take a good screw driver to start it and strong fingers to pull it up.
This will remove the sprung loading on the entire assembly.
Then slide the components apart to inspect for damage.