Some have argued that Brown did not believe in a Pre Mark redaction that he only attributes the difference n GPet and Matthew to Gepet copying from Emery not from the real Ms. He does say he thinks the author copied Matt from memory not that there is no pre mark source.
A disinterested party (Jason Pratt Looked it up for me in his copy of Brown here are his findings :
Joe, per our phone call before lunch,
I went back to pp.1305-1310, which discusses the GosMatt tomb-guard story (or stories) in relation to the singular tomb-guard story in GosPete.
The general gist of Father Brown's argument here remains the same (including with his own reference to Appendix 1's overall discussion of GosPete): GosPete's author imported the guard story from (at least) two sources, those being his remembrance of reading/hearing GosMatt (but not having a copy at hand), and his remembrance of hearing someone else talk about a form on the tomb guard story independent from GosMatt's.
The citation you're probably thinking of, comes from pp.1305-1306, as Fr. Brown introduces this subsection.
"I have argued that Matt [i.e. GosMatt's final author/redactor/editor/whatever] broke up a consecutive guard-at-the-sepulcher story to interweave it with the women-at-the-tomb story, while _GPet_ PRESERVED THE ORIGINAL CONSECUTIVE FORM OF THE GUARD STORY. [my all-caps emphasis] That does not mean, however, that the _GPet_ story is more original.
[...soon afterward on p.1306...]
"In this particular instance, in my judgment, what is found in _GPet_ [concerning the tomb-guards] is best explained in terms of the author's knowing the canonical Gospels (perhaps by distant memory of having heard them), especially Matt, as well as an independent form of the guard-at-the-sepulcher story, and of his own activity in combining these two sources of material."
This suggests that Fr. Brown thinks GosPete's other source for the tomb-guard story traces back orally behind GosMatt to a shared source of some sort, thus that some form of the tomb-guard story predates both authors.
Footnote 50 ends with something similar: "I would suspect that these [verbal similarities to usages of terms found only in GosMatt's tomb-guard story but nowhere else in GosMatt] are elements that Matt found in the original guard-at-the-sepulcher story and that therefore the author of _GPet_ could have also found them there without depending on Matt." This sentence makes no sense unless Fr.B is thinking of GosPete's oral tradition accurately reporting a shared prior tomb-guard story prior to GosMatt.
This has nothing at all to do with a pre-Markan Passion Narrative so far, however. At most he's detecting, through source criticism (via some redaction and form criticism), a tomb story predating GosMatt. Which no one anywhere denies, even among hypersceptics, so far as I know. Even the most radically late JMythers don't think "Matthew" invented the empty tomb! (Although who knows, maybe there are Griesbach or Farrar proponents among them somewhere. That would be sort-of amusing. {g})