In humans, enlargement of internal organs (including heart) is know to be a side effect of using growth hormone as a PED. Wiki says [0] that "Extracted growth hormone was used since the late 1950s", so in 1973 it was already a well known technology.
[0] https://en.wikipedia.org/wiki/Growth_hormone_therapy#Extract...
No, Occam's Razor suggests the simplest possible explanation. To suggest that they figured out how to dope horses in 1973 and then promptly forgot how for the next fifty years is a complicated explanation. The simplest explanation, the Occam's Razor explanation, would just be that it was a fluke of nature.
Maybe anti-doping rules enforcement became stricter?