Here the original manuscript is extant, and it reads guard, not guide. Oliver Cowdery, when copying to the printer’s manuscript, accidentally misread guard as the visually similar (and semantically plausible) guide and initially wrote guide in 𝓟. Somewhat later, with a sharper quill, he crossed out guide and supralinearly inserted guard; the ink flow for the correction is uneven and weaker in some spots, which suggests that Oliver made this correction while proofing 𝓟 against 𝓞. We note that this error is visual and not phonetic (at least as far as Oliver’s dialect is concerned), unlike the phonetic, nonvisual error of meet for beat nearby in verse 22. 𝓞 shows signs of phonetic mishearings, while 𝓟 shows signs of visually miscopying.
Summary: In Alma 57:29 𝓞 reads guard, not guide (Oliver Cowdery’s initial error in 𝓟), in the relative clause “which was appointed to guard them down to that land”.