Wait are you saying that in addition to baffle plate differences, there are also differences in the actual radiators?
Also were you able to test and see if the late baffle sounds the same as the mid baffle if you plug up the holes (maybe with electrical tape)?
Stupid me, the differences in the radiators are the fact that the early ones are orange.
I haven't tested whether plugging the holes makes the later style baffle sound like the middle one, but based on my current observations I have no reason to suspect that it wouldn't. Everything else appears functionally identical except for those four pinholes. My sextetts, like most, have the front grilles stuck fast, so they do not come off easily. Because of that, and now with these latest findings, I do not have any interest in going through the trouble to test sealing the pinholes. I'm going to operate under the assumption that sealing them will change the sound back to the middle style baffle plate, unless I hear otherwise.