The Selectors changed around the beginning of 1969.
Originally, your amp would have had a 100pF (a brown colored RS silvered mica) bypass cap, so the red one that's in there currently is a replacement of an unknown value.
In general, I kind of automatically assume that the 11 series amps started towards the end of January 1968 at the earliest. I've seen a fair share of 10 series 50w amps (forerunner to the 11 series) with D7 & A8 Mullard codes, January '68 filter caps, and February '68 pots which tells me Marshall was still using up earlier production panels into 1968.
No, your reading the pot codes correctly. Who knows why the 1 Meg pots were changed out. The Selectors may have been changed out because the earlier style are more prone to having the plug part fall out. In the one photo you can tell the Selectors were changed out because the mounting screws do not project beyond the nut. The earlier style Selectors used a shorter screw since the Selector thickness was less. Most of the early 11 series amps do tend to have the JTM50 type filtering scheme dispite having a solid state rectifier. So, a 1-inch diameter 32uF/500v filter cap for the mains and a 32uF-16uF/450v cap for the screens & phase inverter.
