In his letter to William E. McLellin, April 18, 1880 (ten years before the first manifesto), Joseph F. Smith wrote the following where he distinguished the “principle of” eternal marriage and polygamy, showing he did not think they were one and the same:
I do not care to argue with you about
the character of Joseph Smith, I always have had and still have the highest
regard for him and his works, and not the least of these the great and
glorious principle of the Eternity of the marriage Covenant,
including a plurality of wives. In my judgement had he died without revealing
these, then indeed his mission would have been a failure, but putting all
together from the first to the last and I see a perfect organism and a grand
design in the revealed purposes of god.