The original manuscript here reads “now behold”. While copying from the original into the printer’s manuscript, Oliver Cowdery initially wrote would. This error may have been the result of misreading the now as the visually similar wou and then combining it with the end of behold: now behold > would. A little later Oliver caught this error and crossed out the would and supralinearly inserted behold (with a slightly heavier ink flow, perhaps after redipping his quill). But here Oliver neglected to also insert the original now.
The phrase now behold is very common in the Book of Mormon text, occurring 149 times elsewhere in the text. Although 114 of these other occurrences also have an and (“and now behold”), there are 34 occurrences without any other preceding connector (that is, simply as “now behold”), as here originally in Alma 30:51. There is also an occurrence of “but now behold” (in Mormon 5:18). The critical text will, of course, restore the original instance of “now behold” here in Alma 30:51. For another example of the same loss of now (but from an instance of and now behold ), see under Alma 32:5.
Summary: Restore the now in Alma 30:51: “now behold he hath shewed unto you a sign”; Oliver Cowdery accidentally omitted the now when he corrected the initial would that he had written in 𝓟 (would was probably a visual misreading of now behold ).