You are right, there was a change from 5-pin to 6-pin (from grey to balck) when MULTIPLEXintroduced the HF-moduls (HFM3) with "channel-check" option.
I think it was launched with the last version of the mc3010 (at least I have one of these) but also was applied to the late mc3030s (not on mine). Even - I'm not sure whether all - the mc4000 (alt least mine) got the 6-pin socket.
With the mc 4000 MULTIPLEX introduced the channel-ckecking feature that ensures you that your chosen channel is free when you switch on the transmitter - otherwise the HF-modul does not work. In the mc4000 the function was more elaborate, offering the checking of the complete 35MHz or 40 MHz range, depending on the module in use.
The feature "channel-ckeck" was "invented" by MULTIPLEX and seemed to be a nice idea to avoid frequency-collisions and so MULTIPLEX wanted to extend the feature to the mc3000 series. But ist was a smaller version without software changes.
Unfortunately the feature did not find its way into the market: only MULTIPLEX offered the system, the competitiors did not. And then came the introduction of the 2.4GHz-Systems (MPX needed some time with the M-LINK-system, but the Graupner-trouble showed they were right!).
The later 6-pin moduls work with all MPX-modul-transmitters, even e.g. the ROYAL-modul of the end 70s. But vice versa you must drill a little hole (about 3mm) for the 6th pin (in fact it is the 5th).
However the channel-check works only with the late mc3000s/4000s. Applying an HFM3-modul works only when the special channel-check modul is insertet. Attention: there are at least two versions of the channel-check modul: for HFM3-moduls and for the Cockpit-SX and the Royal-EVO.
Oh, I got into taking again ...