Haha, I have often suspected it, when reading a wretchedly-proofread reprint. Actually when I saw those SF Gateway editions, I wondered for a moment if it was a coincidence that scans of a couple of those books had appeared on Internet Archive slightly earlier.

I checked the Oxford first edition of Mark: just the first word of each chapter is capitalized, so clearly not that one! And then I looked at the other Front Street editions of the same vintage; Frontier Wolf does the same "capitalize the first line" thing as Mark, but The Shield Ring and Knight's Fee don't. Nor does the Chicago Review Press edition of Sword at Sunset. If neither press imposed uniform formatting, I guess that supports the theory that they just copied each book from a different previous edition.