Originally posted by Ben_Edict These lenses were usually produced with an original T2-thread at the rear, to adapt them to almost any camera. This T2 thread has the dimension of M42x0.75. Can you measure at least the diameter (which should be 42mm) of the thread? You simply need to find the original rear and then fit a T2-K-mount adapter there.
It is well possible, that somebody fitted (well, forced) an T2-M42 adapter on the original rear and later a T2-K-mount adapter was squeezed onto the M42-adapter. This is possible as T2 and M42 have thew same thread diameter, just a slightly different pitch. If that would be the case (an obsolete T2-M42 adapter mounted), it would easily explain the out of focus images at infinity.
Ben
These old 500/8 were sold under many names and some were very sharp in the center.
Thanks Ben, width of thread is approx 41mm (close enough to 42mm.) Length of rear tube is approx 89mm - not including thread.
There are no obvious signs of forced fitment - I.e. the black coating on the threads are still very much intact. Perfect even. HOWEVER, the OTHER end of the rear tube (that is, the end that connects right underneath the tripod mount) does NOT fit very well into the next section at all. It will screw on, very poorly though (I can push them together if I apply a tiny bit of force, ignoring the threads altogether), as I believe that the diameters/thread pitches are slightly off. Must be only very marginal though as I can still tighten them together at the end of the thread, enough to hold the weight of my SLR (though I am worried, and will probably use locktite on the thread just to make sure once I've got this issue sorted out).
Weird
I guess this could mean that the rear tube is not original and it could be slightly longer than the original (hence my reasoning for putting the rear tube length of 89mm above)
Here's a pic of all pieces including the rear tube in the middle.