All factory parts were identified with an attached part number. Small trim pieces came in a cardboard box with the number on the out side. Blocks and heads and any other cast part had a casting number with a date code on the piece that allowed it to be identified. Hardened steel parts like bolts, sway bars and springs had a pice of colored tape attached to the part, all sheet metal and other mild steel parts were stamped with a die identifying the part.
In your case the original paper tape is long gone. Though the springs were not painted and the only corrosion protection was a black oxide finish (a form of rust that protected the part for at best half a year from further rusting) so they would appear as rusty metal today. Yours appear to have been powder coated or painted with an epoxy paint, which isn't stock. Who made the spring I have no idea. Factory used Moog as a vendor for most springs for cars; heavier trucks got a Moog or an Eaton spring.
Springs are identified by their wire size, coil diameter, spring rate, their free height, and the way the end of the spring was treated (ground flat, twisted into a pig tail, or left open). You can easily measure all of the attributes, except the spring rate which you can measure if you can place the spring in a holding fixture and apply weight on top of it to determine how many pounds of weight it takes to compress the spring one inch from the starting point. With this information you can order replacement springs to match what you have now.
Old springs sag with age so even though the wire size and the spring rate do not change, under the weight of the car given enough time the free height changes lowering the car from one to two inches from when they were new.
The factory spring rate was very soft compared to a Corvette or a Camaro SS or a Chevelle SS. A Trans Am raced Z/28 had a spring rate approaching 517 pounds in the front coils. A full size car was closer to 230 pounds. This is why a Trans Am racer can be seen carrying the outside wheel in the air on a tight turn with the car body flat. Compared to an Impala that would have sparks coming off the lower valence on the inside corner side was being ground off by the asphalt as the car body was about to roll over (the opposing outside tire would be hanging down rolled under at maximum extension, but not offering any traction).
If you want to improve your autocross times you have to install a massive anti roll bar front and rear, and stiffen the spring rate, as well as installing wider tires on stronger than stock wheels, which will adversely affect your ride comfort. You can turn a full size Chevy from a land barge into a corner carver, but few do as they buy the car for interior room and ride comfort, not to race it. I moderate this board not because I restore old cars, but because I had a pair of B-bodies that I actually raced (Warning they were sleepers, or a wolf in sheep's clothes that would embarrass a Mustang or import car). No one else on the Team owned a full size car so I was volunteered to moderate this board in the beginning. I owned a full size car because I couldn't fit in a Camaro due to my size, but I wasn't going to accept the stock full size car performance so I changed it so it out ran and turned inside my Camaro owning buddies.