Black Walnut as stated. If you are going to make some repairs, try and locate some "old" walnut - by that, I'm referring to some that is recycled from use many years ago. I'm in MI where black walnut once grew in fairly good quantities. I owned a custom millwork/cabinet shop and had a customer bring me a truckload of boards one time that were actually used as roof boards on a house built in the 1830's. All were at least 14 to 18 inches in width. I recycled it into clocks, cabinets, etc. - something like that would work well as it might match in better than something that is cut from something that hasn't had a chance to mellow. Unfortunately, what I had is all gone or I'd send you a piece of it. If you have to use something that is newer, just try and find a piece that closely matches the grain pattern. Walnut, like many hardwoods, will vary in density and grain pattern from tree to tree. The majority of the Springfield stocks that I've had the opportunity to look at through the years are made out of pretty straight grained material. i.e. not a lot of fancy burls, etc.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63