This is a very interesting story, and the book 'The Planet Pluto' by A. J. White published by Pergammon Press in 1980 gives many estimates. here are some:

Year      Author         Mass in units of the earth
----------------------------------------------------
1931      Pickering                 0.71 
1931      Nicholson                 0.94
1932      Bower                     0.06 - 0.09
1966      Halliday                  0.14
1968      Duncombe                  0.18
1968      Kaladze                   0.07
1971      Seidelmann                0.11
1974      Baldi / Caputo            0.0138 to 0.004
1978      Christi and Harrington    0.0017
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The story ends in 1978 with the discovery of Pluto's satellite Charon which allowed the definitive and final measurement.