This is covered in earlier blogs, here it is again-
"There is the secret monopod that comes with all Tiltalls since the Uniphot (Fred Albu) in mid-1980. A convertible idea - tripod with head changed to a monopod WITHOUT A HEAD. You had to carry an extra head.
"One of the legs unscrews and joins to the removeable stop at the bottom of the column. Stop has a 1/4-20 threaded bolt (Marchioni and Leitz have the stop - usually hollow - but never with the thread.)