Dover isn't your best buy if you're looking for Mahler orchestral scores. This is absolutely true of the Seventh Symphony, which Dover reprinted from the original pressing, with its several hundred errors. This pair of reprints isn't nearly so bad. The score of the Fifth is good: there are one or two misprints, but nothing really crucial. The Sixth however is another story. Dover reprints the first edition of the score ~ which is useful ONLY if you're interested in the history of this score, & then you will supplement it. Mahler subsequently issued a second edition, which was replaced by a third edition, both within months of the first edition. The third edition is the one that has been played & recorded from 1906 through the early 1960s, when the awful critical edition was released, & the critical edition is essentially the third edition without the transposition of the two inner movements.
What are the differences between the first & the third editions? Mahler rescored the work so that there are differences of scoring on almost every page: to take one example, the final chord of the first movement in the first edition uses a bass drum & cymbals. These are gone in the third edition. Between the first & the third editions, Mahler reversed the order of the two middle movements, deleted the third hammer blow, & altered the performance instructions on every other page. As just one example of the last, at cue 80 in the Scherzo, where the Scherzo section is repeated for the last time, in the first edition Mahler doesn't instruct the performer to return to the original tempo of the Scherzo (in fact, there's no tempo instruction at all), so that if one is going to perform this as it stands in the score, one reprises the scherzo at the slow tempo of the section before. This is clearly an oversight that Mahler corrected in the third edition, with the instruction Tempo I. subito. & in the bars around this cue, the instructions in the third edition are either not found in the first edition at all or are different.
The third hammer blow is a crucial deletion. I happen to like the third hammer blow, but my view is that these two editions are different & shouldn't be conflated. If you want the third hammer blow, you perform the first edition; if you want to perform the third edition, you do without the third hammer blow. What do these hammer blows mean? That's anyone's guess. It would seem that Mahler started out with five hammer blows: three where we find them in the first edition, the fourth at the first ff of the introduction, & the fifth at the corresponding point in the return of the introduction. Did Mahler delete the third hammer blow (of the first edition) because of some superstitious fear? We don't have his word for it, & if he did start with five hammer blows, then it seems quite unlikely; all we have is Alma's testimony, & Alma is an absolutely unreliable witness. Not only did she arrogate to herself the office of final interpreter of Mahler's works as if they were her property (& work), but she makes assertions that are contrary to fact & simply impossible given the biographical details. So much of what she says about Mahler's music generally, seems like so much fabrication. It's much better to understand Mahler on his own terms & forget about what Alma says. So it would seem that between the first & the third editions, if Mahler did start out with five hammer blows, he decided that the third remaining hammer blow really lacked the structural justification of the other two hammer blows. In any case, the version that Dover has reprinted is historically interesting as a first & subsequently discarded version of the work, but of no interest for the performing history of the Sixth & for what appear to be Mahler's final intentions regarding it. One hopes that there will be a new critical edition of this work that follows a defensible editorial policy.