The series filters have NO thread on them, they have a plain metal mount. I have a full set for the Leica CL lenses, they simply drop into place in the front recess of the lens and are held in place by the Lens Hood. The lens hoods for the CL lenses are 39mm dia, but a different pitch of thread from the usual. You can use the normal 39mm filters with extreme care, as they will screw in for about a 1/4 turn and just hold, but if ANY force is used they will jam on the lens and be nearly impossible to remove without damage to the lens. The recess in the front of the lens has to be deep enough for the plain rim of the filter which is about 4mm deep.
P.S. I just tried to use them on a CV lens, by holding them in place with an empty 39mm filter mount, they don't fit as they are too deep to allow the filter to screw in to retain them.
