The black plastic part looks like a Betor style used on the later rupps but without slots for a spanner wrench. will the lower cup spin or does it feel tight/fixed? withought any slots they dont make it easy to grab to uscrew there, if they notch cut the cup though you may be able to put a strap wrench directly on it.
Either that or pull the top bushing out of the eye and see if the rod is treaded through with a screwdriver slot :shrug: