FYI:
Frank Arnold recognized my description immediately and sent me this:
"Deccasette should be TU60, see:
http://www.iivs.de/home/fm.arnold/Fotoos/DEC/TU60/DSCN0870.JPG
http://www.iivs.de/home/fm.arnold/Fotoos/DEC/TU60/DSCN0871.JPG
http://www.iivs.de/home/fm.arnold/Fotoos/DEC/TU60/DSCN0872.JPG
http://www.iivs.de/home/fm.arnold/Fotoos/DEC/TU60/DSCN0874.JPG
And yes I know this beast pretty well, more than I liked in the old days.
DEC made CAPS-11 "operating-system" especially arount this peripheral,
basically the papertape programming software on casettes.
I'm not aware of any surviving copy of this..."