But, as Ben Zimmer notes in his 28 April post to ADS-L, one of the (now four) newspapers known to have run that particular strip published a version in which “butthole” appears as illegible or partially so. Ben found this case in newspaperarchive.com’s reproduction of page 11 of The Zanesville (Ohio) Times Recorder. The entire page is now viewable at the URL below. Interestingly, it’s just that one word that’s particularly difficult to read; even the teeniest print on the page is easier to make out.
So, perhaps – as Ben suggests – someone at the Zanesville paper was familiar with an offensive meaning for “butthole” and decided to have the word obscured before that day’s issue went to press.
Further, as Ben points out in another posting, Green’s Dictionary of Slang records a 1942 usage of “butt-hole” to signify, well, “anus,” so at least the hyphenated form was known to some segment of America as a synonym for “asshole” when “butthole” appeared in Montana’s strip.