#theres a whole section of tables of physical and orbital data

[piratesexmachine420]

Some interesting excerpts from the Rand McNally Atlas of the Solar System (1983, ISBN 0-528-83102-X):

Pluto [...] Orbit and Nature Yet before long, several disquieting facts were established. First, Pluto is not a giant planet: it is certainly no larger than Earth, in which case it can hardly exert any measurable influence upon giant planets such as Uranus and Neptune. Second, Pluto's orbit is unusual. Its eccentricity -- 0.248 -- is greater than for [sic] any other planet, and when near perihelion Pluto comes closer in than Neptune, although a relatively high orbital inclination (17°) means there is no danger of a collision. Pluto's mean distance from the sun is 5,900,000,000 km, but this ranges from 7,7375,000,000 km at aphelion to 4,425,000,000 km at perihelion. The next perihelion is due in 1989. Between 1979 and 1999 Pluto temporarily forfeits its title of "outermost planet". The period of revolution is 248 years. The small angular diameter is very difficult to measure. For many years the true status of Pluto remained a puzzle. It is also extremely difficult to determine mass of Pluto. [...] Origin of Pluto Pluto's small mass means it cannot possible be Lowell's Planet X, because the perturbations this body could produce in the movements of Uranus and Neptune are far too slight to be detectable. It is even likely that Pluto is unworthy of planetary status. There have been suggestions that it used to be a satellite of Neptune, which broke free in some manner and moved off in an independent orbit. Triton, Neptune's satellite, has a strange, retrograde orbit that could also have resulted from this incident, and both Pluto and Triton are believed to have frozen methane surfaces. However, the discovery of Charon weakens this theory. Alternatively, it may be that Pluto is the brightest member of a whole group of asteroidal bodies in the outer part of the Solar System. In 1977 Charles Kowal discovered Chiron (see pages 244-5), which moves mainly between the orbits of Saturn and Uranus. Other small bodies may well exist, although Chiron is smaller than either Pluto or Charon and radar measurements indicate that it is rocky rather than icy in nature. Planet X? [...] In general, astronomers believe that Planet X may exist. Certainly Pluto does not adequately explain the mysteries of the outer Solar System, and it may even be regarded as a double asteroid system rather than a planet. [pg. 396, 397]

Emphasis mine. Keep in mind that the IAU conference which eventually reclassified Pluto wouldn't occur for another 23 years!