I think the MK III's were first produced around 1943. Honestly I would stay with the MK III, the PSU is quieter and a better set overall. You won't get the loud humming noise with the MK III
And really the only difference you will see is the PSU power to set is a 12 pin instead of the 6 pin used on MK I and II's. And the label that says MKIII/date.
I have not seen any type of throat mic with 19 sets, I collect 19 sets/ancillaries and have never came across a throat mic for them.
Most of the throat mics are with portable small sets, 38 set etc.