Alex,
I have an old Army Special (made in early 1903) in good mechanical shape, though not quite of collector quality. I do not believe that the 41 Long Colt was a "rare" chambering. Prior to WWI it appears to have been relatively popular. Ammunition was produced on a regular basis well into the 1950's, Winchester made a special "run" within the past 15 years.
The 41 Long Colt "double action" cartridge was produced in two distinct configurations. (Note: there was a distinct 41 Long Colt "single action" cartridge, not for the SAA which used the 41 Long Colt "double action" when so chambered, but for the Colt "New Line" series of revolvers, but that is a different story).
The original version of this cartridge had a case length of about 0.935", and used a heel bullet. This load was replaced, some sources say in the early 1890's, by a load using an internally lubricated hollow base bullet placed in a case having a nominal maximum length of 1.130". To complicate matters, most early revolvers have land diameters of near 0.410", but by not long after the turn of the century barrels having the same groove diameter of the 38-40 (~0.401") were more or less the "rule" .
Inside lubricated ammunition for this cartridge doesn't shoot as badly as its reputation would have it. Use of either late production Western ammunition or handloads using Lyman 386178 (an inside lubricated hollow base bullet) cast 1:30 gave 25 yard groups of roughly 3¼" in my Colt Army special.
Outside lubricated ammunition loaded with Lyman 386177 (a heel type bullet) was a disappointment at first. The cylinder of the old Colt was too short to use with the heel bullet loaded into cases of the current standard length, so I shortened the case length to the old standard. Accuracy was not good, to say the least. Looking for the trouble, I decided that the forcing cone was far too small for these bullets. I recut it to the same dimensions I'd use on a 41 Magnum. With this modification the revolver is capable of 2½" groups. Back off the powder charge recommended in the old Lyman-Ideal handbooks a bit for this application.
I hope that this has been of some help.
Bob
Anyway, if anyone's interested, I'd part with it for a few $$$ and put you in touch with the guy who is supposed to have the plugs, maybe you'll have better luck.