Bruno Walter actually conducted three performances of M7 in 1920. They took place on March 5, 6, and 7 in Vienna, at the Grosser Musikvereinssaal. At first only two performances had been announced. The first on March 5th was billed as a “Festaufführung”, to be followed on the 6th by a “Viertes Zykluskonzert.”
The first performance on March 5th was later re-billed as a public dress rehearsal (“Hauptprobe”). It was then announced that due to demand an additional third performance would be added on March 7th. A notice in the “Neue Freie Presse” for this performance may be seen here, near the top of column 3:
http://tinyurl.com/387van8Walter’s arrival in Vienna to begin rehearsals was noted in the “Neue Freie Presse” on March 3rd, which means he must have arrived a day or two before that.
It has been difficult to find many reviews of these performances with the resources available to me. In a Feuilleton which appeared on March 8 (signed “J. K.” = Julius Korngold), three paragraphs were devoted to a review of the music and performance. Only the last paragraph deals directly with Walter’s conducting (in the Finale). I attempt to translate it as follows:
“Bruno Walter brought out this last movement with the most sensitive feeling, using a dynamic which saved on breath, wisely restrained, and scattered the light. This excellent artist really approaches all Mahler-music with such a complete, singular understanding! The knowledge and feeling that he possesses stood him in good stead with the Seventh as well. Full performance fortune bloomed for this piece, which in previous Vienna presentations was abridged, but not this time. A shortage of rehearsals could not be overcome. The lines were perfectly, clearly drawn out; the spirit was present but the timbre stayed away, not to speak of flu-affected bells and tenor horn. All the more wonderful then was the Mahler-loyalty of the audience, all of whom showed their readiness, even towards this weaker work of the Master, to provide feelings of appreciation”.
For those who can read German, the original text may be found at the bottom of column 2 (from the “Neue Freie Presse”, March 8, 1920):
http://tinyurl.com/2vr8rrwAssuming my translation is correct, Korngold implies that there were cuts (or not all movements played?) in previous Vienna performances (there can’t have been very many between Mahler’s death in 1911 and these Walter performances).
In the combined April 1920 issue of the “Musikblätter des Anbruch” (2nd Year, issues 7-8), there is an article entitled “Representative Mahler Performances in Vienna”. I translate the very brief comments by the author (R. S. Hoffmann) as follows:
“…Also this year, he [Walter] fascinated us with the powerful ‘Seventh’, with which he unbelievably filled the hall three times.”
Mike Bosworth
I've never seen a reference to a B.W. Mahler 7, so I'll just have to take your word for it. Regardless, we have no idea how it sounded. Thanks.
It's not my word, it's De La Grange's (I'm not such an authority-not yet! 
). We have no idea how it sounded, but we could have a little bit of it if he could trace the score that he used (for example, he used to jot down timings and he did so even when he followed Mahler's interpretations).
Luca
