The U&N had converted to air brakes by 1885, so almost all of the cars received by the SV from the U&N would have had Westinghouse Automatic brakes. Did you happen to measure the car? Length and height of side sill. My guess is that it is at least 27' long. If it was a former U&N car it most likely was a product of the UP shops Omaha or U&N shops Eagle Rock.
Tapered side sill? If so you have a flat or coal car.
Straight side sills, former box or stock car, or possibly a 20th century SV flat.
Ron Rudnick