I would say save your cash and buy a book on FS instead.
It really is not (*usually) difficult to spot copies or fakes, if in doubt, simply post pictures of the one you have found on the myriad of FB collecting groups, commando knifes, FPM, Fairbairn Sykes etc., before you spend a penny.
*When you get into the Nail Spike handle, or very rare FS variations, some costing $4000-$5000, the fakery is museum quality, but you're talking 3rd patterns and other than idiots stamping 1942, cross of Lorraine signs, and other stupid obviously spurious stampings you're fairly safe.
I also collect trench knives and US fighting knives and these are starting to become a minefield of fakery.
The Indian one on WWA is a horrible copy of something that never existed, a fat-boy grip with wonky etch - its not military issue anywhere, it's a recently made fake to deceive novice collectors.
The 1975 NATO marked is Ok, but it's a govt. contract version of the cheapest and worst variation of the post war style of FS.
Stamped guard, machine ground blade, unnumbered hilt.
Fairbairn didn't like the WW2 3rd pattern variation, he would have hated the post war 3rd pattern.
Buy a book, read-up, the way that the FS knife was developed is an interesting and historical story. Then go to Ebay US, or any of the collectors sources (first pattern militaria, Ivan Gamsby) and just wait and watch patiently.
I don't think I have spent over $200 on an original WW2 3rd ever, I keep meaning to stop buying them, as I have enough, but they keep coming up in auction.
Usually seller spells the name wrong, mis-titles it, doesn't know what it is.
They make good trades when making deals on rarer knives.
I once traded 10 X 3rd pattern knives for a 1st pattern.
We were both very happy.
Anyway - the above is my opinion, I do not know you, or what your collection is like and can only project myself into the situation you describe, so I sincerely apologize if I have offended or talked out of turn.
My grammar sometimes rubs people the wrong way until they know I mean it all for the best.
When I stopped buying anything that caught my eye and invested in reference books my collection changed profoundly.
Two First Pattern Fairbairn Sykes flank an original 1918 LF&C Trench Knife.
The 1918 still shows it's original blackening on the blade and brass.
https://www.milsurps.com/images/impo...RxFuFCol-1.jpg