Threads on Lee Enfield aperture rear sights

Hi all. This has stumped me so I'm posting here to see if anyone can help.

I've got a BSA No 9 aperture sight fitted to my long Lee.
Fitted to it I've got single aperture eyepieces that screw in.
I recently bought a multi aperture eyepiece to use instead that is marked up 'Parker'. However.... This screws in about half a turn then locks up.

So, up until this point I had assumed all BSA (made by AG Parker / Parker Hale), Parker Hale and AJ Parker sights used a common sized thread. Physical evidence is now telling me otherwise, can anyone shed any light on this please?

The matter of screw threads used by Parkers, Parker Hale and all of the others is a total enigma and has been featured on this forum since time began. I personally believe that they did this so as to make their sights totally unmanageable, unfathomable or repairable by the average simple bloke like me armed only with a simple 3 (or 5) year apprenticeship and a degree in engineering. Their screw thread system doesn't seem to equate to those found in the usual engineers reference books or....., anywhere really outside the factory drawing office.

My sons old school Cadet Force had a stack of the sights together with a letter refusing to sell them spare parts or screws but suggesting that the sights be returned to the factory for refurbishment as some considerable cost. A nightmare

P&H cranes did the same Peter they made all the seals a different size than what one could buy over the counter along with main components so you could not get a part from another supplier it had to be a P&H genuine part if it was not genuine it did 2 things did not quite fit or if it did fit was the wrong thickness and would leak under pressure. Motto - cornering the market.
Liebherr cranes make pretty much the whole deal from the ground up except for a few off the shelf components and are considered me included after operating a 160 Tonne one for 4 years they are the Rolls Royce of the cranes. Vid clip ~ To prove a point at a show awesome there are certainly load moments and compression forces at work in this display.

Liebherr Crane Mobile - Customer Days 2012 - large.wmv - YouTube

Awsome. Did GROVE cranes ever feature in commercial Australia? The RAE's had some up in the North as I seem to recall. Only later when I was back in England did I realise that they were made a couple of miles from where I live and later I was offered a job in their technical publications/writing office.