Ive stocked a pile of shotguns from blanks, only 1 rifle. I can steer you to some resources and give you something of a basic rundown if you want, but not sure what you are looking for. Are you interested in using it to stock a gun yourself, have someone else do it, or? Assume it’s a 1-piece rifle blank, do you know what kind of walnut it is and what the history of the blank is (ie age, air dried, etc)? Post a pic if you can.
I don't have any tools for stock making and don't have a build that I want to do. It will probably never get used if I keep it. I'm unsure about the specific type of walnut that it is but it appears to be well seasoned. I will get some pics and measurements.
Hard to tell from the pictures, but it looks like a nicely colored straight-grained flat-sawn blank. Yours appears to have good grain-flow which makes it strong, that's the #1 priority. On first look it appears to be American black walnut, but I've seen some english walnut that looks really similar to this as well, so without seeing it in person I wouldnt bet too much either way. There was a guy I think in texas maybe 15 years ago named j earl bridges who sold a bunch of fairly plain and reasonably priced "tessier" french walnut blanks (english walnut supposedly from France that was known for having a very rich red color)--I'm pretty sure he was a gunsmith for many years and retired or stopped stocking, and got rid of a large inventory of stock blanks. I bought a couple and used them on some shotguns, it was not super fancy but really great wood to work with and those blanks actually looked pretty similar to yours, so for all I know this blank could be part of that lot--probably not, but you never know. There are some darker-smoky-looking areas on the blank that make me think it's possible this could be english walnut, but hard to say from my screen. Odds are it's eastern black or claro walnut, which are more typically that reddish color like that.
In any case, assuming that is pretty tight-grained wood, if it's a black walnut blank it is not as desireable because it is usually heavier and harder to work with, but it's also more appropriate for specific guns. English walnut is typically most desireable, with claro more variable and somewhere in the middle. A nice blank is air-dried for many years to make sure it is fully dried without having tension built-up in the wood that would cause cracks or splits as it's worked and used, so good stock blanks are very expensive compared to a piece of regular walnut lumber. A really fancy, air-dried figured walnut blank can go for many hundreds of dollars up to 4 figures easily for highly figured english walnut. Standard-grade wood like you have is obviously less$, but can be hard to find a good blank since most good stock blank sellers dont spend a lot of time on anything but the high-$ stuff. If you were to buy that blank from someone who specializes in gun stock blanks it's probably a $100-150 piece of lumber if its eastern black, maybe $150-250 if english? Depending on what you used it on, you'd spend another $1500-$2000 to pay a gunsmith to put your action in it and finish and checker it. If you have a family gun that's trashed and the wood came from a family member or very close friend, it could make sense to do something like that. Usually, once people see the price tag or amount of work that goes into stocking a gun, they often shell out the extra couple hundred$ to use an uber-fancy piece of wood, but a blank like yours to me looks fantastic on an older field-grade gun, whether that's a shotgun or a rifle. (you could stock either a shotgun or a rifle with that blank). If you wanted to do it yourself, you'd need a gun and a lot of time, and a few hand-tools--a vise, a couple chisels, maybe a couple specialty hand tools, a couple screwdrivers, a drill, some inletting black or a lamp, and lots of grit paper, but you can do a professional job stocking a gun with $100 worth of tools if you take your time. You can also send an existing stock and the blank to a stock-duplicator who can duplicate the older one 95% leaving you with just the final fitting and finishing--this is less intimidating and easier although still a lot of work. There are a few good books and some hobbyist websites where you can get very good info on this along the way.
If you want to sell the blank consider finding a local gunsmith who might use it and trading for some work, or you can list it for sale on ebay (fraught with people trying to pass off random lumber as gun stock blanks so beware) or list it on a website such as doublegunshop.com or a site catering to folks who like custom or vintage guns with wooden stocks. A gunshow will often be a decent place to sell it as well. I'd expect if you arent able to answer specific questions about the wood you'd get people lowballing you if you try to sell it. You may have best luck at a gunshow or in person where someone can see it.
Pic below is a gun I stocked using a piece of that tessier walnut--that blank had a bit more figure than yours, but it gives a sense of what you might expect that wood to look like after being finished. Eastern black walnut and claro walnut would usually be a little more open grained and each look slightly different, but you get the picture.
At some point it's like the difference between a roma and a beefsteak tomato--they have slightly different characteristics and they aren't the same, and while one may be 1.99/lb and the other is 2.99/lb, they're both tomatoes and you can make salsa with either one. If you were going to sell it and try to get really top dollar it'd be good to know, but if you or someone is looking for a stock blank with that sort of wood grain and color, it should work great. So you may never be sure unless someone knowledgeable can look at it in person, and it likely doesn't matter really. Hopefully that helps, but if you have any specifics I could try to help with, fire away.