Help. Duplex junkie needs another fix

I am living with three HP inkjet printers that I bought used for next to
nothing. They all use the same color cartridge and all work perfectly
with the duplex attachment that came with the HP 970c (I believe that it
was originally bundled with this printer). The other two printers are HP
940c and Photosmart 1115. One printer is in daily use, another is in
occasional use connected to a second computer. The third one is stored,
as yet untested.

The savings of paper, printout bookshelf space (and a little in cost),
are very welcome after a couple of decades of one-sided wasted paper. I
became addicted to the duplexer very quickly. I cannot imagine living
without duplex printing!

But now I'm paranoid. What will I do if my beloved duplexer
self-destructs? So, I'd appreciate ideas about finding another one to
stash away as a spare. For starters, I'd like to know how to identify
the one that I have now: model number, etc. Also, what to look for if I
find one lurking in a thrift shop -- with or without a printer attached
to it. And, perhaps, which printers originally came with them. Or any
other useful advice.

TIA.

Richard