I think that flat is spurious. The evidence is that the other similar patterns do not match bar 31. In 26-28, and again in 30 and 32, beat 1 harmony is repeated in beat 3. So bar 31 could be different on purpose, but why?
In the Ditson edition, which was probably issued soon after the original in France, there is no flat on that note:
http://petrucci.mus.auth.gr/imglnks/usi ... 51nuns.pdfThe original French edition is not on IMSLP. I assume the Cb is in that score and replicated by the modern editions.
So, my bet is on an engraving error (as it seems was Ditson & Co's) , but you could go either way. I play that pattern to match the others.
Erik