|MadSci Network: Astronomy|
In fact, the mass of Pluto was not known until several years after the discovery of Charon in 1978. Early estimates were too high by a factor of several hundred. These estimates were based on apparent perturbations in the orbits of Uranus and Neptune, but the perturbations were the result of observational error and not actual orbital anomalies.
I have a couple of old astronomy textbooks in front of me. The first, An Introduction to Astronomy by Robert Baker, published in 1940 (ten years after the discovery of Pluto), lists physical data for all the planets but does not attempt to provide a mass for Pluto. The second, Introductory Astronomy and Astrophysics by Elske Smith and Kenneth Jacobs (1973 edition, and the textbook used in my first college astronomy class), estimates the mass of Pluto to be between 0.01 MEarth and 0.18 MEarth based on gravitational perturbations, on size estimates from a stellar occultation, and on albedo. The currently accepted value for Pluto's mass is 0.002 MEarth.
The fortuitous orbital inclination of Charon between 1985 and 1991 resulted in a series of eclipses that made it possible to measure the orbital period of the moon and the sizes of both bodies. From this the combined mass of the system could be calculated. Even today, however, there remains a relatively large uncertainty in the actual masses and diameters of Pluto and Charon (as compared with our knowledge of those values for other planets). We may not get much better values until a probe is sent to Pluto.
A couple of good references on Pluto can be found at Nine Planets and wikipedia.
Try the links in the MadSci Library for more information on Astronomy.