Best to post pictures or at least give a detailed account of the markings.
But to be going on with -
1) the rear of the receiver should be milled out under the rear sight ie with the sight folded down there should be daylight visisble from the side.
2) the butt should have a rubber buttpad with a metal frame and recessed area on the right for the sling.
3) under the woodwork the know form should have two scallops of metal removed.
Speaking of these,I've observed one on G.broker that has all the correct features of an early Faz,but the date on the rec. is 0/44! Can't make out what I would surmise should be a 1 in front of the zero,for October.The electro penciling is also too fresh looking over a well worn finish and patches of stove paint.I'm wondering about this one,don't think I'll bid....
Just making the point that it is not a good indicator of "original".
I have a bin full of #5MK1 original rear sights that I got from Century years ago. They have gone up in value and are pretty easy to buy now. Numrich and SS has them. As far as I know they have never been 'scarce".
Also, just for interests. I have seen every imaginable incorrect part on real #5MK1s. So the thing to determine is if the barrelled receiver is original. As posted by others the lightening cuts are a sure way. The stampings on the barrel and receiver are usually also a good clue.
Recently I have seen a lot of orignal #5s with replaced after market wood, one piece or all.
Also, unumbered hollowed out bolt bolts are easy to get. As well as any other parts to make fakes look real.
To be precise, lightening cuts only show that the barrel and receiver are the real deal. It is conceivable to find one with a real barrel in a #4 receiver.
To be sure one would have to check out every part on the rifle to see if it is ALL original.
I have seen lots of "fake" #5s with correct rear sights and hollowed out bolt handles as well as other parts that look very close to the real deal.
The trigger guards for a real #5MK1 are quite a bit different than #4.
Wasp waist contoured and the part that fits over the trigger is not as wide as one for a #4.
I have seen several variations of trigger guards. I have two that posted for sale on a recent WTS post, both came off a #5 rifle but is a little different on the front. Very unusual.
I'll post a picture of it. Maybe someone will recognize what variation this is. I am curious. I'll have to post the picture on a seperate post below.