I do wonder if the stock pads would have sounded better and been more comfy when new, the foam generally is a victim of degradation over time. . . Koss are definitely hit and miss on comfort, but I am increasingly of the opinion that their stock seal and clamp are quite deliberately designed for the headphone.
I suspect you are right regarding the origins of these following the ESP/10. I would think it was a manufacturing strategy to sub in the more popular dynamic elements as the ESP10's were going out of production perhaps. That said, I can't track down a year of release for these, so I suppose that the possibility exists that the ESP/10 reuses this cans enclosure rather than the opposite.
the stock pads are air-filled vinyl pads. no foam in them. i suspect these pads were quite similar to the pads of the ESP/9, which were liquid filled as far as i know.
i found a website which provides some basic information about them...
http://20cheaddatebase.web.fc2.com/koss/DYNAMIC10.html
the translation sometimes acts up so i'll attach a screenshot as well. these were released in 1977 and discontinued around 1980.
also found a short article from Billboard magazine: https://books.google.com.ph/books?id=S0UEAAAAMBAJ&pg=PT32&lpg=PT32&dq=koss+dynamic+10+billboard&source=bl&ots=iorLXS3zZI&sig=YCZqQn6NDEP7-V4NqEnTvuRF_5c&hl=en&sa=X&ved=0ahUKEwj_t_nPrpfOAhUBkJQKHdEHDpsQ6AEIIDAA#v=onepage&q=koss%20dynamic%2010%20billboard&f=false
the article highlights the release of the ESP/10 and briefly (one sentence) mentions the release of a "dynamic version" (at $85).
spent a little more time listening to them yesterday and am really liking the sound. it seems too that they sound best with instrumental jazz and funk from the same era. they don't sound as good with more modern music. "PYT" (Michael Jackson) sounds GREAT on these. also noticed that there's A LOT of sound leakage. the whole headphone resonates. i can hear them from across the room! the cups i think are made from steel/aluminum. i'll consider damping them in the near future and see what happens. i haven't really modded any other headphones aside from Kramer modding my PortaPro and KSC75, and have yet to research on the possible benefits/effects of damping sealed headphones. just hope i don't damage them in the process...
UPDATE
i haven't opened these up yet but i'm planning to:
1. line the insides with Dynamat or similar to isolate the drivers from the cups
2. apply a second layer of felt/acoustic foam/fiber fill to absorb backspeaker resonance
what effects can i expect from these "mods"? i'm basically "winging it" here. no science involved.