NGC 4157 is shown above with Zwicky's 1937 supernova arrowed.
According to his article published in the Publications of the Astronomical Society of the Pacific:
Before anyone knew what a galaxy was it had been the convention call them "nebulae". Today the word "nebula" refers to a cloud of gas in space. At the time of his writing Zwicky knew that the "nebulae" he was referring to are all what we call galaxies. Edwin Hubble's discoveries about galaxies was less than a decade earlier and the term was a holdover from that previous age.
From September 5, 1936, until the end of May, 1937, about 300 photographs were obtained covering as often as possible the Virgo cluster of nebulae as well as its northern and southern extensions into Coma Berenices, Canes Venatici, Ursa Major, Hydra, and Centaurus, respectively. In addition, some of the nearby nebulae, such as the great nebula in Andromeda, Messier 33, 51, 80, 81, 101, NGC 55, 247, 253, 2403, 2366, 4236, IC 342, 1613, etc., were frequently photographed. It is estimated that, in the period mentioned, between 5000 and 10,000 nebular images were carefully searched for new stars.