Pawn and Two: <backrank> It is hard to determine whether 26.a3, or 26.c3 was played.
I checked a couple of other sources. The book, <"7 Matches 1905">, edited by A.J.Gillam, included the Janowski - Marshall 1905 match, with match introduction, and extensive game notes from, <"The Match and the Return Match: Janowski v. Marshall">, edited by Leopold Hoffer.
Hoffer's title, <"The Match and the Return Match: Janowski v. Marshall">, refers to the Janowski and Marshall matches of 1905 & 1908.
Hoffer shows move in question to be 26.a3.
Hoffer's only comment at move 26 was to note: <"if 26.Qxh2 then obviously 26...Rg6+ 27.Kh1 Qe4+ wins">.
My Fritz database, and the database at 365 chess, show the move to be 26.a3.
By move 26, White is lost, all moves are clearly losing. By using computer analysis some interesting points are revealed, but this analysis does not resolve the question whether 26.a3 or 26.c3 was played.
If White played 26.a3, Fritz indicates the best contnuation is: (-11.32) (20 ply) 26...Qe2 27.Rf1 Bf4.
If White played 26.c3, Fritz indicates the best continuation is: (-14.80) (20 ply) 26...Qe2 27.Rf1 Be5.
This analysis does not help to resolve the question, the game continuation followed the computer continuation for two half-moves after 26.a3, and for three half-moves after 26.c3.
If 26.a3 was played, then Marshall missed a quicker finish with 28...Rf4! 29.f3 Rf5 30.Rf2 Rg5+. Also, after Marshall's 28...d3, Janowski could have held his hopeless position a little longer with 29.cxd3 or 29.Nc3. Janowski's move 29.f4, allowed a forced in 11, with 29...Rg6+, or in 13, with 29...d2 with 29.
If 26.c3 was played, then by move 29, Black had a forced mate available against any move by White.
As you have indicated in, <"Marshall's Best Games of Chess">, former title, <"My Fifty Years of Chess">, Marshall showed the move to be 26.c3.
Maybe somebody will uncover some additional information to help resolve which move was actually played.
It is interesting to note that Marshall concluded his notes to this game with the statement: <"This was considered the best game of the match">.
Hoffer's conclusion was a bit different: <"There is hardly a single of Janowski's games in which he has shown such poor judgement of position. Marshall played splendidly.">