Before Frankfurter (1970) with Frankfurter Highlight (1978), there was Pluto. This rounded sans serif was issued by Face Photosetting in 4 styles; the outlined Pluto together with Pluto Shaded in 1969, followed by Half-Solid and Solid styles in 1970. I don’t know if Letraset’s creation of Frankfurter was influenced by Pluto, or if Face quickly added a solid style to Pluto after having seen Frankfurter. Or maybe there wasn’t a direct relation at all, and the sausage style was just in London’s air at the time. What’s certain is that Letraset adopted the original outlined style for their dry transfer lettering range as Pluto Outline in 1972, under license from Face.
Unlike the original (heavy) Frankfurter, Pluto has a lowercase. Its caps follow the classical proportions more, as exemplified by the narrow P vs. the wide H. Frankfurter looks like a more balanced revision of Pluto Solid, with several improvements made to the distribution of black and white, see e.g. the lower A bar, the shorter middle arm in E and F, the larger bowls in P and R, or the narrower U and V. The outlined and shaded styles with their three-dimensional look are where Pluto really shines.
The cover of Panther’s paperback edition of Do Androids Dream of Electric Sheep? from 1972 pairs Pluto Solid and Half-Solid with Helvetica. The cover art is by Michael McInnerney (b.1944), who’s probably best known for his cover of the Who’s album Tommy.