Are they really old? they look to me as though they recently arrived from Pakistan via a mail-order catalogue.
I suspect a poorly made thread on the cuff, and the thread in the key might not be well-made either. It will be difficult to remove the riveted endcap to dismantle the lock. Tilting the key as it is turned might help it to grip the threaded screw. Otherwise,it might be a paperweight or display item.
