Merrick wrote an autobiography in 1997 - fascinating reading, but only a few copies around. There's a short section on recordings that lists these (which includes some reissues and compilations):
The Old Chisholm Trail (Riverside) Songs of the Old West (Washington) Singalong! Folk Songs for CHildren (with daughter Kate) (Birchmount) A Child's Introduction to Folk Music (Wonderland) An Introduction to Folk Music and Folklore (Wonderland) Ontario Lumberjack Songa (CBC) Australian Bush Songs (private) Cowboy Songs of the Canadian West (CBC) Cowboy Songs for Little Cowboys (Golden)
Recently Merrick made private recordings "Old Time Religion" and "A Man and a Maid" with Jean Mills, and his new album of cowboy songs, which I am going to try to encourage the family to make available, if any of you are interested.
All of these were out for viewing at his memorial yesterday - quite an amazing bot of history.
Kat - Merrick's Old Chisholm Trail was kind of his theme song, and his friends called him "Muk". I wonder if by any chance he is the "Mac" that you have on tape?