I have seen a couple of those "no name" shotguns like you describe, but I have no idea who made it. (Those dates are 1891, 1900, of course, not 1991 and 2000.) I think if you really want to, you will have to make both the screws and the spring. The Brownells screw blank pack should help on the screws. I would talk to the customer and see if he really wants the gun fixed, as making parts is expensive and the gun is not really worth it. If he tells you to go ahead, ask for a hefty deposit up front. He will probably change his mind when he realizes the cost involved.
I say this because it is common for guys to bring in old guns that "belonged to grandpa" and ask to have them fixed "for sentimental reasons." After you put a couple of hundred dollars in parts and labor into a $25 shotgun and give them the bill, they forget about "grandpa", tell you where you can shove the gun, and walk out.
Also, the gun may well have a damascus or laminated barrel, and if so, I don't recommend firing it.