Hi, since your original post, i have obtained an Ariston RD80SL, and like yours it is missing the original clamp.
I can see now that the universal clamps offered by other manufactures will not work with this deck. The turntable spindle has hardly any plain diameter, just enough to locate the mat and not really enough to centre the disc. the rest is threaded.
I have, in a seperate thread, asked if anyone has a clamp, could offer dimensions and/or confirm the thread which is an oddball. No replies, so it would seem that we are on our own.
Machining a clamp is not a problem for me, and after a little research have determined that in my opinion the thread is a 'truncated' version of an M7 x 1.0. Truncated (which means that the top of the thread form is removed) probably because a sharp new thread on the spindle would enlarge the centre hole of the discs. I will now estimate the size of the clamp, produce one and give it a try.
assuming success, Dizzybixbexter, if you are still in need, send a PM.