Since nobody seems to know, I just ordered two of these arms. They are compatible with ULCS arms! I paid only 7.50EUR per arm, in the US they are even cheaper (6.95$ at Adorama). A rare gem - an item for uw-photography that does not cost a fortune!
got two 8", will (intend to) use with net floats as they fit well in the central hole on most floats (10-12 mm). just worry about ball-rod joint. this arms are made not as a solid thing, alike ULCS (INON, Nauticam...)