You should probably read Riggs' stuff and/or check out his podcast to see what he's saying his sources are. We know from Peterson's
Game Wizards that most of TSR's official sales records are still retained by WotC, and as I recall those archives (as well as court records, and correspondence held by private individuals) were one of his primary sources.
Underpinning the business story of Game Wizards is the financial model shown here of TSR as a company, from the founding of TSR Hobbies i...
playingattheworld.blogspot.com
Which "all sales"? He's clearly labeled what he's counting on the charts. For the Basic/AD&D comparison, as discussed, Riggs is comparing all the editions of the Basic set to sales of the AD&D PH & DMG from '79 to '95.
That's the thing. WotC CLAIMED that, but they didn't have all the information, ESPECIALLY of the early days from what I know. That wasn't kept overall in the main offices.
We have accounting information and invoices of sorts from some of those who were working at the time, but they didn't have the accurate records at TSR as of 1985, and probably thus did not magically gain them later from what I know. Much of it was reliant on the memory and some information that some of those in the financial department have posted or put out in other locations.
Even in the 90s it wasn't what I would say was the best kept set of records. That's probably a BIGGER factor in the TSR bankruptcy than many would think. A reason they were spending more than taking in at times was because of ignoring blatant things on the record sheets...but also some of those things just weren't making it anywhere and sometimes one could say some of them just magically...disappeared. Who knows where they went by the mid 90s.
I've seen records where in the early 90s TSR actually broke 100 million gross income, and right before that it was around 90 million (give or take one or two million I think) in gross income (a fact bragged about, I believe by Williams at some point, which also shows the growth as well as drastic change from TSR in the 80s, which at a high I believe took in around 26 million).
Now, if his numbers don't JIVE with those...something doesn't JIVE between what those who have stated things previously about working there and various numbers, and what he has found. Of course, those numbers didn't jive at times with what WotC said when they were trying to tear down TSR and show they were much better than TSR either.
The early records weren't held by the Williams I don't think. If I had to guess, they were directly held by the Gygax's and their fellows. I don't think Arneson was invested enough at the time (in tracking the actual financial records and paperwork, even if he was interested in the money, he was more an idea guy from what I gather) to really bother with the record keeping, thus it was mainly on Gygax's side of things. It could be that Kuntz is helping him and is the source. There ARE other sources that may be able to supply the information (though I think it may be ethically questionable, if not blatantly illegal, and in that case, they probably would NOT want to be stated as a source out in the open) that is somewhat reliable. How he convinced them to relay that information is...well...WOW.
The VERY LEAST I'd expect to see on his sources are requests from the SEC for the old tax filings of TSR, if nothing else. At least see if he attempted that, and if he got lucky, those documents themselves. That would be legal. Official sales records from the various companies selling the game at retail would be good sources of information. (Maybe he has gotten them...the things I am seeing though that are red flags are that the numbers he has posted don't match up to previously stated numbers from what I would consider rather reliable statements. Someone said he didn't have sales numbers for international sales, which could be where the discrepancy comes in, but if that's not a factor, something doesn't match up in regards to the numbers. Graphs look fine without the numbers in their representation for what I know, but the numbers is where my eyebrows are raising).
Maybe he got the gold mine of all gold mines, but that's something not even WotC got from what I saw. It's why it has been so hard to piece together some of the early financials of the days at TSR, because they LITERALLY could not be found by the normal means (or at least legal means...there WERE some paperwork...and I believe that's what WotC looked over, but it's not ALL the paperwork from what I know).
PS: Good article by the way. External Audit is a good source...as long as...once again...he got it from the OFFICIAL sources. I think that would be the SEC that could release it if they wish, otherwise there would probably still be needed a Gygax or someone close to them to authorize the release, at least ethically if I understand the standards of personal information of audits vs. that sent to the SEC.