As you have surmised, I also believe your stock in not original. The photo you’ve just posted does appear to be an original Model 30S reflecting the “improved” later “NRA” stock configuration. I've just had a quick refresher look at my own photo file regarding two of my Models 30 straddling your rifle serial number. One of mine is in 26xxx serial range and the next closest to yours in 28xxx range. The earlier one wears the first generation stock with the schnabel foreend. The latter one, in the so called "NRA" factory style and is marked as a "30S", is much closer in appearance to yours. BOTH utilize a prominent cross bolt just under the receiver ring. (I believe such is also visible in the photo you just posted.) There are also other details which differ between my NRA stocked model and yours that suggest it was not a factory stock customization. However, mere photos and senior eyesight don’t make for a positive conclusion without more. Yet the cross-bolt omission seems conclusive. My 30S does also wear a grip cap of hard rubber or phenolic of some kind and it bears the Remington name.
As to value, a frank answer. Your rifle appears irreparably modified from factory original. It is thus largely without collector value and simply within the ‘nice old shooter’ category. Unfortunately in such category, it joins a sea of such rifles. As with my own nice old shooters, they appeal to me. (Or at least did for some reason at purchase!) But objectively, they generally suffer in the market due. I would suggest your rifle might value around $400 to $500 and that attributing some value to the aperture sight. Of course a supporting hands-on inspection and knowledge of your particular regional market would be required to reduce the variables. In my SoCaSea area where black rifle fever now seems contagious, it would likely be on the lower end of that spectrum. Yet all said, sometimes guns do speak to us and I happily own more than a few in that category.
Enjoy your rifle!
My take.