This one is a little worse than it appears in the photo. The border between the checkering and the back strap is almost worn flush.
Worn yet still sound hard rubber stocks
This is the best I can come up with. I had a pair of stocks on a 1953 NYPD Official Police revolver that were pretty chewed but I "upgraded" them with a cleaner looking pair. The old ones are somewhere in my boxes of stuff.
As to what causes it, exposure to any and everything even minutely abrasive on a regular basis. That would include hands, jackets, other belt-carried equipment, a slung long arm, car seats, chairs, walls, door frames, the ground (during scuffles) and so on.
The 4-inch Smith & Wesson Model 10-6 Heavy Barrel shown above was purchased used from a armored motor guard that made deliveries to the dock of the very large bank where I worked as a very young teller. I ran the dock and handled all incoming and outgoing shipments of cash along with other securities. The guard wanted a "prettier" nickeled plated version of the same revolver so I gave him $75 for his blued revolver. Shortly after the acquisition I discovered that I could have purchased the same gun brand new for $78 which shows what an astute gun trader I was. Hey, when you're only 18, and lusting for the first handgun of your own you take advantage of opportunity when it knocks.
Our dock was off the basement of the parking garage and ran back up under the bank building. It was very narrow, allowing only a skinny ledge on either side for the guards to navigate as they loaded and unloaded their trucks. To a man, their Colt and Smith & Wesson .38 Special revolvers had deep flat spots planed into the wood of the stocks from dragging against the brick wall that comprised the sides of the dock area. My acquisition was shipped in 1971 and I acquired it in 1975. It had scarcely been used but the right stock was a wreck which I replaced. Of course it was mostly the right stock on those revolvers that looked that way since the majority of the guards were right handed. Any lefties would have exhibited the same problem only their left grip panel would have been banged against the sides of the trucks.