THAT splice just happened to be where the stock took a bullet, got a chunk knocked out somewhere along its way, or sustained damage in some other manner.
Stock repairs can be, and are, literally anywhere & everywhere on individual Mosin samples.
Yours is unique in the location of that "splice" only in the sense that it happens to have a repair in that exact spot. It wasn't a mass-produced thing, it wasn't a model or year-specific thing, it's just a repair.
The repair in itself adds nothing to the rifle beyond a bit of character.
I may be mis-reading your posts, but if you're thinking your rifle is a "rare", as in desirable or valuable, variation solely because of the replaced section, it's not.
I've seen Mosin stocks with little sections of wood fitted in here & there all over.
Those with original factory spliced-on sections at the butt end were done on new stocks, little (and even not-so-little) inserts elsewhere don't add any desirability as re-furbed repairs.
Denis