I'm going to make some statements as if fact but please, argue with me as this is all stuff just gleaned from the web.
In general:
Older tubes have square getters.
Older tubes have black plates.
Any 6DN7 with acid etched dotted code on them were made by GE regardless of any other markings.
I've got 9 (one blew up) 6dn7 tubes with dotted code on them. Some labeled GE and some labeled RCA.
As reference for the GE date codes, both as labeled on the base (if there) and the dotted codes:
http://www.antiqueradios.com/forums/viewtopic.php?f=19&t=230932and as a reference to the coding on the RCA tube bases (table on page 18). The few tubes I have marked RCA seemed to follow that table when compared to the acid etched dots.
http://pax-comm.com/rcadates.pdfIn short - the acid etched codes are the date of manufacture. The label on the base was for warranty purposes. I still haven't been able to completely figure out the acid etched dots on some of my tubes. Some make perfect sense in that what the acid etching has is always the same year or earlier than what the label on the base has. I have one tube with no markings on the base at all.
Tubes dated 1958, 1960 and 1962 are black/grey plates with square getters.
Tubes dated 1964, 1965 are black/grey plates with round getters.
Tubes dated 1966 and after are grey/grey plates with round getters.
Is the smaller of the plate construction the cathode? And the other, more complicated structure, the anode?
Patterns on the mica vary also but not between tubes close in years.
Cathode are all grey. The oldest tube I have has holes in the anode structure.
It's interested that these changes were made. I'm wondering if anyone has reference as to why? Most folks I know prefer older tubes - black plates with square getters but I'm not sure if that's just a status or nostalgic preference? As we all know production lines get simpler and material typically cheaper (whether for good or bad is not part of the reason - cost and profit are) so it might follow that the older tubes are better. But does that give them a "sound" advantage?
I also have a pair of Sylvania coin based tubes. These have 1/2 mica's that don't go all the way around the tube. A square cathode and a very flat and simple anode. Both plate structures are black. I'd say these are the newest of the 6dn7's I have but I can find no date code info on them. (NR AAA and DR AAC)
Sound - I'm not sure my old ears can put one above the other...but I'll put the oldest ones in every time anyway!