Old stock, unused cartridges:
Price 7 EUR + shipping 9 EUR
i have a few for sale
email me at dedoogunNOSPAM@@@NOSPAMgmail.com ( remove NOSPAM of course and @@@ is one @ ;) )
Thanks
[WTS] 16K SRAM cartridge with RAM-disk BASIC extentions TECALL TM220
Yes, i was curious and opened that one i will keep for myself (it has case with screws so it was pretty easy to open):
and after reading the manual i think that the bios of it is quite non-trivial :) especially the kind of key-value store for basic variables
So far: 13 cartridges were sent in three parcels (one was 1kg and has 10 ones, thanks @hamlet), 2 are reserved, 3 or 4 available (i haven't decided yet if i keep two for myself or only one) 
UPD: two reserved ones already paid while i was writing message
I actually got this device from Japan in a working but quite used state. Amazing that you found these unused! Where did you get these??
I've never seen them anywhere else anyway... I'd be happy to get such a new one to join my weary used one I hope some of these that came to Europe would be available
They have special commands for working with files, like CALL RFILES, CALL RSAVE, CALL RLOAD etc, so it not a disk drive emulation, and thus no boot sector - not possible to boot DOS.
P.S. But i wonder if games which can store your progress could be patched to use this one for saving data
