I might be wrong, but I believe they were handwritten in the submitted manuscripts then manually typeset by the publisher.

In Feynman's PhD thesis, all of the formulae are written in:

http://cds.cern.ch/record/101498/files/Thesis-1942-Feynman.p...

Tediously! See: http://ultrasparky.org/school/pdf/Rhatigan_Monotype_4-line_m...

This thesis actually describes a scheme Monotype developed to replace the older hand-setting approach (used through WWII). Nice intro to the subject though.

Preston