That could possibly be the one, but it doesn't seem like it because I remember it being a darker wood, but I'm sure the one in the picture is just an oddity.
Nah, the one in the picture is right, and so is your uncle's. It isn't used as often today because more and more firearms are being made from metal and high tolerance plastics, but Cozmoline is a very effective grease-like substance that is used to store firearms that use wood in their design, it prevents normal decay (since most rifles of the period were never coated with a protective wood finish), it also prevents insects from destroying the wood and it helps keep the rifle mechanics from rusting.
Unlike normal greases and gun oils, Cozmoline is used for very, VERY long periods of storage. When the K98 rifles were produced they are normally a very light yellow color, which comes from the wood they were made of, over time the Cozmoline will impregnate and dramatically darken the wood. Cozmoline was very heavily used by both Germany and is still used in Russia, which is why many AK-47 and SKS stocks feel sticky even after a heavy cleaning.
In fact this is also one very common movie mistake, even RO made this mistake, but I dont care because its a small error.
But if that little picture looks like the same design as your Uncle's rifle, then you got a match.
As for my take on cheating and taking advantage of little goof like Diving with IS, I really cant say why many people use them, maybe its a way to compensate for somthing, maybe they get a feeling of power, or they really are just that bored. All us honest players can do is keep trying our best to get it fixed, and luckily for us, TW gets it done quick.