Arguably the episode in which Rhoda invites Mary to her sister's wedding could be considered something of a pilot as it looks like a rough version runthrough of Rhoda with a few retools to come, not the least being switching Married Debbie for Single Brenda.

http://tvtropes.org/pmwiki/pmwiki.php/Series/TheMaryTylerMooreShow?from=Main.TheMaryTylerMooreShow