Alf Svensson for Fritz Hansen chair advice.  

Well I am thinking that I'm just going to have to have all the plugs fabricated. I haven't heard a quote yet, though, and so I am not sure if it will be reasonable. If I have to order like a thousand, does anybody here want some?

Nice chair mgee76, and yes, it looks like oak. Without seeing it in person I couldn't guess if the finish is original or not.
If you don't want to go to the expense of having brass plugs fabricated from scratch there are alternatives. We once bought a Dux chair that had previously been reupholstered. Obviously at some point one of the brass screw covers had gone missing and somebody made one of wood and painted it to match. We did not even realize it until after we'd bought the chair, and even after we did it was no big deal. The wood plug served the same purpose and looked just fine. You really had to scrutinize the chair to see the difference. Plus, if you have new brass covers made they won't have the same patina from age as the originals, they will look shiny and new.

Yeah I was thinking I would tackle the patina issue when I came to it... I suppose I could make some from wood and paint them, but hrm. Thanks for the suggestion! Would definitely be cheaper.

It looks like maybe a lacquer finish...? Lacquer chips off rather than wearing off evenly. The wood could just be dirty where it is chipped off. The light parts might be scratches that are deep enough to go below the surface patina which has just darkened with age---maybe? It's hard to tell from the one photo.
I think you can darken raw brass right away with certain stuff, like maybe bleach. I know bleach will turn silver dark immediately. But if not, it will go dark pretty quickly anyway as long as it isn't lacquered. A lot of brass stuff is lacquered and that's why it stays bright and shiny for a long time. (Former brass instrument player here--there were people who thought a lacquer finish affected the sound of a brass instrument but no one wanted to play an instrument that had to have the icky-smelling tarnish polished off all the time.)

Darkening brass, bronze, copper is not difficult at all if you use the right chemicals. Patinating non-ferrous metals is an art arguably perfected by Edo-period Japanese armorers that could produce a wide range of colors.
The most common agent used to darken brass is a liver of sulfur solution, but small quantities of various pre-mixed solutions are available in the US by Birchwood-Casey from gunsmith suppliers, gun shops, and online retailers. I use them regularly with excellent results. Just follow the simple instructions!

Thanks for the suggestions... I'm pretty sure I could fashion some replacements out of clay or something similar that would work, but I'd much rather have the real thing. I'll get some quotes on making some and see what happens... I'm sure I'll have plenty to test the best method of aging with.
Spanky, here is a better picture of the wear to the armrests. I feel like it's lacquer? Don't really want to strip it.