Charles C. Imus came to California from Illinois in 1846 as the co-leader of a small group of pioneers. One of the members of the party was Adna Hecox, Santa Cruz Lighthouse's first lightkeeper. For a portion of their journey the group traveled with the doomed Donner Party.
Mr. Imus was the son of Hiram A. Imus Sr. (not the Hiram on the Imus obelisk https://sketchfab.com/models/56db612f25c241688e4dc810ba9580bd) and his wife Ruth. He died May 13, 1856. It seems rather strange that there is a rectangle carved between May and 1856, particularly since it is not centered and seems too short to have been a blank for the day. The stone is marked as being from Excelsior Marble Works, San Jose (http://quarriesandbeyond.org/states/ca/ca-stone_carvers/e-5_excelsior_marble_works.html).
For more on Charles and family see:
http://researchforum.santacruzmah.org/viewtopic.php?p=603&sid=29baaf21ed266e38aba184612224eee5 and
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&db=rdwyer&id=I32