Jeff: On the 65-66 SJ pads, have you seen variances in the surface appearance? I've seen some that look 'flat' and some that are 'shiny/smooth'.
Most of the time there is a light or thick coat of dust and little particles of the padding that have stuck to the visible side. If clean and at the right angle they have a slight shine to them - more of a semi-gloss that I believe is what they looked like when installed originally. You can often find the protected area behind the heater that is shinier/smoother than the exposed area just above the heater for example. If the particles have been on the surfaces for a long time they can bond themselves into that side (possibly in those very hot 120 degree and higher) where its almost impossible to get them all out. I think this is what can also contribute to the flatter and less smooth (not lessening the dimple pattern) visible side.
Some of this may just be a result of poor quality control during manufacturing resulting on the difference. So for me - bottom line there are allot of things that cause what we're seeing and its likely a combination of one or all.
Couple of pictures comparing dirtier and cleaner examples
Dearborn/Metuchen surface texture/pattern