Interestingly, the 1960 ACC lists T206 as having 522 cards, and it also explictly lists O'Hara and Demmitt as being worth $1 each. In an appendix it says "Value 25c for cards No. 4, 208, 242. 50c for cards No. 168, 172, 347a. $1.00 for cards No. 104, 148, 383. $10.00 for Plank No. 143. $50.00 for Wagner No. 368."
At first I wasn't sure what checklist Burdick was using. It's not Frank Nagy's T206 checklist, which had appeared by at least 1971 and which I posted in this thread:
http://www.net54baseball.com/showthread.php?t=203207
Nagy listed 524 cards. He had Wagner as #367, one off from Burdick, but Plank as #293, nowhere near Burdick's number.
Then I looked in Lew Lipset's Encyclopedia of Baseball Cards, which cross references each T206 card with its number in three previous checklists. Apparently Burdick was using the list in the American Book of Checklists, which was popularized by Rich Egan and known as "Egan's list". It lists cards in team order, starting with all the Boston AL players in alphabetical order, then the other AL teams, then the NL teams, then each minor league. Nagy's list was the basis for the checklists in the early Sport Americana (Beckett) Price Guides, which listed all the major leaguers in one alphabetical list, then all the non-Southern minor leaguers in one alphabetical list, then the 48 Southern Leaguers in an alphabetical list.
Lipset just listed all the player in a single alphabetical list, as nearly all checklists today do, but he only listed 523 cards. The one card from the standard 524 that's missing in his list is Joe Doyle (N.Y. Nat'l). Lipset discusses the other standard T206 rarities in the accompanying article, but not that one. He discusses the "Magie" variation, but says that it was not well known as late as the early 1970s, when a collector found a cache of T206s while on vacation that included seven Magie cards, and sold them through a hobby publication at only a modest premium. Magie is included in Nagy's list, but maybe that and the Joe Doyle variation are the two missing from Burdick's 522 in 1960? I'm not sure what the other card would have been that was missing from Burdick's earlier total of 521.
Edited to add: I see that Dave Hornish put much of this same info in his post, which he posted while I was writing this one, except for Joe Doyle (N.Y. Nat'l) being missing from Lipset's list. I see now that the same card is missing from Egan's checklist, but then how did he get to 524? Hmmm. I don't have time to figure it out right now.