Historical note:
I have looked into this in two old code books for determining if an existing 1935 installation was up to the code in effect at the time.
The 1934 code section 2005 and 1940 section 2105 lay out rules for 15amp circuits.
The rules for 20A circuits were in section 2006 in the 1934 code and section 2106 for 1940. The code only allowed heavy-duty lamp holders on 20A circuits.
I looks like all candelabra, intermediate and medium base lighting outlets were required by the code in effect at the time to be on a 15 Amp circuit.
Where 12AWG wire is encountered in an old house as part of a general purpose circuit that also feeds lighting it would be for voltage drop reasons (which may have been mandatory).
Probably homeowners replaced 15 amp edison fuses with 30A fuses over the years.
Then an electrician comes along and see's 12AWG wire feeding lighting circuits and puts it on 20A breaker during a panel upgrade, not realizing that most of the circuit is probably in 14.
Hence the myth that it was permitted.
The 1947 code I looked at permits medium base key-less lampholders on a 20A circuit by exception, so perhaps this occurs less often after that.
