DMW's comments are correct I believe.
Years ago, I used to work for a local NBC affiliate and recall reviewing the archived press kits (photos) of some of the vintage programs on that network including Bonanza. It was a different era (socially) and "bulges" would certainly have been frowned upon! The earlier on-the-set photos of Bonanza did have the Cartwrights in khaki-type pants (and light-colored ones at that.)
I've always recalled hearing that Michael Landon was brought to our town for a publicity tour (before I worked at the station) and one of the old time staffers told me (and several of us in casual conversation, really) that Mr. Landon insisted (as in HAD to have!) some 'specially arranged evening entertainment. It apparently was quite a challenge in that small southern town to make such accomodations possible but they must have managed somehow. The comment was that there were some positively giddy women (yes--as in SEVERAL of them) that next Monday morning following Landon's weekend visit.
--Of course, I don't mean to be one spreading rumors!