KEG: A fine game by Marshall from beginning to end.
11...Nd7: Marshall here sidesteps an old losing line for Black. The tempting 11...c5 is bad for Black, and if after 12. dxc5 Black plays 12...Bxc5 (better is 12...Be7, but if Black intended this why play 12...c5?) White has a winning combination beginning with 13. Bxh7+ as was noted, albeit with some flawed analysis long before this game in Bilguer's Handbook, since after 13...Kxh7 14. Ng5+ Kg6 15. h4 gives White a winning attack. By contrast, 15. Qg4 here (championed by Bilguer's Handbook, and by Rosenthal in the Tournament Book) throws away White's win after 15...f5 16. Qg3 Bd6 17. f4 Bc5+ (rather than the suicidal 17...Kf6 given in Bilguer's Handbook and by Rosenthal). Marco suggests 15. Qd3 +, but that leads to nothing for White after 15...f5.
Even worse for Black in the above variations is 14...Kg8 which gets crushed by 15. Qh5 (or 15. Qd3) Re8 16. Qh7+ Kf8 17. Qh8+ Ke7 18. Qxg7.
Marshall's move (11...Nd7) avoids this line entirely, and affords him a nearly equal position. Marco suggests 11...Nc6 as a alternative, but Marshall's move is far better.
12. c4: I agree with bishop and with Rosenthal, Marco, and Schlechter that this was not best and allows Marshall to wreck Mason's King-side pawn structure. Rosenthal's 12. Re1 does not strike me as much of an improvement. Best here was 12. g3, avoiding the combination Marshall was able to spring after 12. c4.
18...b6: I agree with Fiendish that this is a "beautiful move." As bishop notes, it "looks like a typo but it is not." As Rosenthal notes in the Tournament Book, had Mason responded 19. fxN then Bxe5 wins for Black, since after 20. Rb1 Bd4+ followed by 21...RxB wins for Black. Nicely done by Marshall!
19...Nc6: Another wonderful move by Marshall. Had Mason played 20. dxc6 Marshall would have played 20...RxB with a winning game.
24. Kf2: Had Mason played the superficially appealing 24. Bc3 Marshall would have gotten the better game with 24...Re3 (but not with 24...Ne2 as suggested by Schlechter).
25. h4: I agree with bishop that this was a mistake (though not the losing move). As the Tournament Book points out, White could not here play 25. Re1 because of 25...RxR 26. KxR Nf3+ 27. Ke2 NxB 28. KxN Bxf4+ (26. BxR is better for White in this line, but loses to 26...Bxf4). Best for White here would have been 25. Rb1 or 25. Rg1.
25...f5: Just about the only weak move by Marshall in this game. 25...h5 was better.
28. Rg5: At this point, Mason seems to have lost the thread of the game. This move loses time (the Rook retreats to g1 on the very next move). Better would have been 28. Rf1 or 28. Rd1. After the text, Mason definitely had the worst of the struggle, but was probably not yet lost.
30. Rb1: The losing move. Marco's proposed 30. Rf1 is better than the text, but can land White in trouble as shown by Schlechter. Best here would have been 30. Rd1.
30...Be7: As Rosenthal aptly notes, this move is a winner.
32. KxB: Though not pointed out in any of the commentaries I have seen, this move was a blunder that allowed Marshall to finish the game quickly. More stubborn resistance would have been possible after 32. Kg2, though Black still had a won game.
After 32. KxB?, Marshall finished nicely.