I use a Watson 66B that has the "key" to close Nikon and Leica cassettes, very handy. I bought that one of from eBay for 9 bucks and it also had some feet left-over film in it. The only marking on the side was "Kodak safety film" but nothing else. I simply "guessed" ISO to be 50 and exposed some frames. One example:
("Kodak Safety Film" / Rodinal / Leica IIIf / 28/3.5 Avenon)