Project Gutenberg does not identify a source text. Presumably its source uses the original long title and short author name, displayed below, although the ebook header does not.
The ebook contains no transcript of the title leaf. Its title screen displays credits, an Editorial Note on apostrophes:
"... The policy of Project Gutenberg is to treat apostrophes as they were in the source text. In this case, apostrophes were omitted in the essay section but used in the play."
and these four lines:
BACK TO METHUSELAH
A Metabiological Pentateuch
By Bernard Shaw
1921
The Contents are "The Infidel Half Century" (an essay in 49 parts, or collection of 49 titled essays) and "Back to Methuselah" (a play, Parts I to V). The essay section spans about one-quarter of the whole length.
Following "The Infidel Half Century", one half-title screen (distinguished by horizontal lines) displays alone:
BACK TO METHUSELAH.
The next displays alone:
PART I—In the Beginning
and so the titles of the other plays are fashioned subsequently. If the transcript is reliable, those features must distinguish the source edition.