So I have an update on this matter and have since rectified the problem. Firstly I'd like to thank Marty for all his help with this matter as he was my go to with questions and the occasional visit to his place to cross reference parts etc. So the thread on the extension housing is in good condition and the thread on the gearstick is also in good condition. For what ever reason the thread in the extension housing is in fact larger than the 41.8mm thread on the gearstick. I'm lead to believe there is the rare occasion some extension housings take a 43mm gearstick. I've since bought a spare Escort gearbox from Marty that my gearstick threads into the extension housing with no issues. Plan is to rebuild one good gearbox from the two with the use of the extension housing that facilitates my gearstick. I've cross referenced all part numbers on both gearboxes and all part numbers match each other. This shows that there is three different gearstick threads for Escort gearsticks, early mk1 Escort type 2 gearbox with a 40mm thread, later model mk1 & all mk2 Escort type E gearboxes with a 41.8mm & 43mm thread. I assume the 43mm thread is quite rare to see as even Marty was saying all his years of experience with Escort gearboxes his never never seen a 43mm threaded extension housing.