2k7 and Lower Volts Introduction?

[RockinRocket]

I have a really great understanding of the 50 watt 1987s jmps.. from component choices to the circuit variations over the late 60s and through the 70s from studing pictures. But two things.. I have a good idea but cant figure out.

I cant figure out when the 2k7 cathode resistor was introduced. Was it just a MK2 thing or was it introduced earlier than that?

When did the lower 370 volts happen? Was it a MK2 thing or a later 70s change when the 2204 cascaded amps came out and they probably started sharing power transformers? Also the same time the 2204s came out is when the 1987s B+ droppers changed to 3 10ks.

Thanks any information would be appreciated!