Thanks Ade for you interest yet again.
It's an interesting little knife. It could be something quite rare or a total piece of junk.
The thing that gets me is the patina and the exactness to the prototype in Roys article in every way even down to the unfinished steel. It appears period constructed and no components have been disassembled to my eye. I worked as a Boilermaker for 20 odd years and the finish to me matches old 1940's-1950's toolage, chipped and greyed with age. I think it's no copy, Roys find was only a few years back and this has age to it.
A couple more images that show this knife has not been tampered with.

Another FS knifeAnother FS knife

The scabbard has been with the knife for a very long time I feel as the copper oxide from the rivets which attach the tongue to the scabbard body has leached into the blade and the split in the leather is due to the fact the blade is around 3mm wider (from memory) than the standard Pattern 3 FS blade. As per Roys example yet again.
Could this knife possibly be another steel handled FS prototype ? You would think more than one example would have been constructed as their were definite recorded balance problems with the Pattern 3, if not by WS ( It does match the WS though) then by another manufacturer.
As it was picked up ex US could it be an example sent to the US for evaluation ? Alas questions with out answers.

It's as I said rather contentious and I fully understand the lack of responses.

All the best and cheers