As I mentioned in the earlier reply, my 1A,that is very similar to yours, has been hunted with since it was purchased new in 1975.  The flaking finish is worst around the edges of the checkering and much like yours over the rest of the stock. As a hunting rifle, it has been exposed to rain more times than I can count.  My personal opinion is that any defect or edge that isn't water tight lets water soak in and results in the finish lifting from the wood.  That is just an opinion, but I am 100% certain that it is the original finish.

I had an early #1 with an epoxy finish that caused pitting at the wood line. Sent it back to Ruger and they scraped and replaced the rifle. It was a 458 tropical. New gun didn't even come close in finish or performance. This was a comon problem with this group of rifles.
Tom