It makes a bit more sense, being for an audio transformer, as, apart from standard "Telephone company" types, which were made in the millions, (& were not wax coated), full sized audio transformers were made in much smaller numbers than power transformers, so may well have used such techniques.
I have seen, used, & messed about with, power transformers from late 1920s units to present day types, & I can't recall any using wax coatings.
Power transformer manufacture is a mature technology, & the insulating materials & varnishes they used back in the 1940s, '50s, & '60s were more than capable of,doing the job they were used for.
Most of the failures I have seen were open circuit windings, or windings with shorted turns.
These are usually from severe overload, which would cook any modern transformer equally badly.