have you tried the lens frontwards yet ?
im not guru, but ... i have a feeling the threaded part is not the back of the lens, but the front ..
it is threaded because the former owner used a barrel that s/he found handy
and put the slider/patched the waterhouse slot ... lens shade screwed on?
you might also have a good time removing the cells, and
mounting the lens in a different barrel, so it is full-open
with no aperture ring ...
have fun experimenting!