This is analysis of the fake rifle compared to two original rifles from 1941 Gustloff production:
UPPER BAND

Look at the original and notice the sharpness of the lines and the edge wear on the corners. Also, look at how sharp and clear the numbering is on the original band. The fake band lacks the sharp edges and high-end wear, the numbering is not sharp, but blurry--it looks engraved.
BOLT BODY

On the original bolt face there are circular striations from the milling operation--this is common for BCD production-especially mid-to-late war. The bolt face has a worn spot from where it hits the rear receiver bridge. The numbers are sharp but uneven, some parts of the number are deeper than others as it was struck that way. There is wear on the edges of the bolt. On the fake, you can see it is flat as they had to grind the surface to renumber it. Still, they did not fully remove the previous numbers--you can see something behind the "2". There is no edge wear and no strike point from the bolt hitting the receiver--again, the numbers look engraved.
RECEIVER TOP

Look at the striations on the top of the receiver--there are fine lines from the swift turning operation. Gustloff receivers were not polished: there is also high edge wear. Now look at the fake: it is polished and bright blue: it's been refinished.
BUTTPLATE

As others have noted, the buttplate should have the number and the suffix. The fake one is missing the suffix--it is also clear the numbers are stamped over something else.
LOWER BAND

This is faked pretty well. If you look at the raised, oval area under the sling bar, you will see the fake is totally white and lower than the original: they had to take material off to renumber it. The edges on the fake are more rounded than the original.
STOCK RIGHT

Nothing on these markings screams "fake" to me and the surrounding finish does not look sanded down and renumbered. Even fake rifles will sometimes use original parts.
TRIGGER GUARD

There are variations among Gustloff rifles because they made some parts themselves and used subcontractors for others. However, for 41 production, the trigger guard consistently comes from the same subcontractor based on the examples I have seen. In addition, on the fake, look at the flat and smooth surface: they had to grind the old markings off and still managed to leave some behind (look at the "9"). The fake E/749 stamp they used is nowhere close to the original in quality.
STOCK CHANNEL

On nice original rifles the inside of the stock will be white like the original one here. With heavy use and wear, it will darken, but the wood in the stock will have a rough feel because they did not sand this area. When you fake them, you have to remove what was there and this often leaves a smooth feel. It is difficult to tell with the fake because the picture is bad; however, you can see there is something behind the "O". Also, if you look to the right you will see a darkened area from heat generated by firing the rifle. This does not naturally fade into the lighter area of the stock, indicating some of the wood surface was removed in this area.
BOLT SHROUD
View attachment 66074View attachment 66075
The fake is clearly over-stamped.
KEEL

Even on high condition rifles, the keel will be dinged because when a rifle was carried by its sling, the keel was the part that was hanging to the side of the soldier and would bump into things. Notice on the fake that the keel is perfectly smooth--not a scratch on it. This is because this area was refinished.
NOTES
There are many other things wrong with the rifle, but this provides enough of an analysis of what to look for to tell original vs. fake. For those of you who do not use it already, I recommend the WW2 weapons forum Auction section:
http://ww2weaponsforum.com/forumdisplay.php?22-Auctions&
If there is a fake, it will typically be posted there and discussed.
If a new collector is serious about learning all of the nuances of these rifles, I recommend they buy the Karem/Steve's books, use the picture library here and most important, build a picture library from the auctions. I obtained all the pictures of original BCD 41 rifles I used here from auction pictures stored on my computer. I organize them in folders in a year/maker/letter block/serial number format. For example, "1941 Gustloff Werke L 4001". Whenever I am not sure about a rifle, I can compare it to known examples in my picture library. This requires an investment of time, but it pays off by not having to learn by buying fakes.
Sources
Gunbroker 173424318
Gunbroker 317203422