Jeffrey Saks at Lamed posts about a supposedly autograph copy of a volume of Mishna Berura with the word מוגה written inside the cover in the Chafetz Chaim's own hand. (I say supposedly since the provenance is unknown, according to Saks).
I showed it to a rare Judaica dealer, who pointed out that in the upper right hand corner of the inside cover is the Hebrew word "mugah" (checked, inspected). He told me that the Chofetz Chaim himself (and his sons-in-law) inspected each volume as it came back form the printers to make sure there were no mistakes in the binding, etc. That notation, "mugah", is none other than the Chofetz Chaim's own handwriting (according to the book dealer).The photo of the inside cover is here.