Leave the edges light, thats what the true colors look like. Practice and just remove it if the first try doesn't look right. Neither the original Case Hardening Colors or the chemical colors hold up well w/o protection. Permanent Markers work or you can add something to the Expoxy/Sawdust to darken it before you apply it: Stain, Cement coloring .
Well used Field grade guns aren't worth a lot to begin with.
It may be worth more if your refinishing looks better than it does now, particularly to you.
It is not advisable to load ammunition to the limit of safety of a shotgun for the reason that the pressures at this high level will ruin the pattern percentage developed by the load."IMHO those Damascus barrel warnings that began appearing on shotshell boxes by the early 1930s were more a thinly veiled attempt to coerce shooters into buying new guns, though they probably did have some relevance to all those cheap Belgian imports that came into North America from 1880 to WW-I. What makes it bad to have the lever to the left when closed. Just remove the first try with the bluing remover if you don't like it. Remove the old finish with Lysol Bathroom Cleaner (it has HCl in it). If it is around the action you can darken it before you apply the varnish to hide it.
Apply the color with a Q-Tip in squiggly patterns like above. You might try filling the missing stock wood with a mixture of sawdust (Walnut or a stained sawdust to match the color) and Epoxy. Many old guns have darken wood from rust reacting with the Tannin in the wood so it shouldn't look too much out of place.
I'd stick to light loads in a light gun; less recoil and you aren't shooting Turkeys.
You can roll up an Index Card or something similar, insert it into the Chambers and mark it to find the Chamber lengths.
If you post the complete serial number, perhaps Walter Snyder, who wrote the books on Ithacas will chime in with shipping information. Unless "special ordered" for longer shells, 20-gauge Flues Model Ithaca doubles of that era were chambered for the then standard 2 1/2 inch shells, which carried a maximum load of 2 1/4 drams of bulk smokeless powder and 7/8 ounce of shot. But this is the light weight little bird gun I've been looking for. I'm not sure how to put pictures on this site but I have a public album on Photobucket you are welcome to view all the pictures I took.
In the pre-WW-I years Ithaca made many of their smallbore Flues Models very light, but 20-gauges weren't that light!! Just open the first picture and you can view the rest as a slide show. ...
Ithaca was having problems with frames cracking on these very light guns, as North American Nimrods were shoving the hottest loads available into them. a Flues/ In my opinion, the cost to properly restock a well worn Flues Field Grade, on top of the mechanical issues of that top-lever well left of center, will greatly exceed what the gun will ever be worth.
Over the years Ithaca increased the weights for their smallbore Flues guns. The S on the watertable above the 20 is normally considered the stamp for a No.
Proper, low-pressure shotshells are available today, enabling many of us to shoot Flues' in good condition, from companies like RST, Poly Wad, Kent, Game Bore & others.