As I understand US law, in the U.S., you have a one year grace period after any public disclosure in which to file a patent. In almost all other countries, you have to file a patent application in one country before you make any public disclosure.
"Public disclosure" is publications in books and technical journals, and also includes poster sessions, slides, lectures, seminars which are open to the public, letters, and even conversations (except with employees who have non-disclosure agreements). Any written or oral disclosure, even to a single person, counts as a "public disclosure" in most countries, but in the U.S. it has to be in some form of writing or drawing or picture. The disclosure must teach someone "of ordinary skill in the art" how to actually duplicate the thing. An offer for sale, even if it does not teach someone how to make the thing, also stops patentability after one year in the U.S., but in Europe selling things doesn't qualify unless they also show someone how to make it.
So, any public pictures of schematics, or photos which show circuitry, which have been available for more than a year without being patented-- are pretty much fair game. This includes pictures on any sites that come up in Google with a simple search. (You can't say "I didn't know they were on the web" if they come up on the first page of Google.)
Obviously, all the mod pdfs and images which have 2003 and 2004 dates on them, etc-- are all fair game, unless some hobbyist actually paid ridiculous fees to actually patent a mod, which I strongly doubt.
It's also been 20 years since the original designs. So unless ADA still has a magical long-lasting patent, it would appear that the original boards, plus the SS mod, Mod 2 Marshall, Mod 3, Mod 3.666, and Mod 4 are all public domain. Finally, since MarshallJMP is selling the "ADA Depot 3TM (3-Tube) Mod" for quite some time,-- unless he's doing it under license from here, under license from Tod-- I think it's also totally public domain.
Three of those mods in a single rack (yeah, I know-- 7+ tubes.... hehehe) would make an insanely useful 3-channel preamp, switchable by footswitch or anything that switches simple jacks. I imagine if I did such a thing, I'd throw in a cheap spring reverb with a tube driver, and a tube-driven effects loop.
As a software developer who uses open source code every day, this excites me all kinda ways. Does anyone have any specific information about claims on this stuff?