Who patented this spool and what part is missing?

Taken off an Underwood from the 1920s, this 5-spoke ribbon spool is probably an aftermarket replacement spool from a later date.

It was originally lacquered black, but the paint on this spool had reacted badly with the ink of the old ribbon and they jointly turned to 'goo'. With hot soapy water most could be washed off, the last remnants removed with white spirit or gasoline. 

A neat typewriter ribbon spool, at first glance not unlike Underwood spools or the commonly found Grafton ribbon spools.

Stamped on the flanges however is the text: PAT'D - this is a patented spool design. No clues however on who made it or on patent date or a patent number. (Or even a hint in what country it was patented.)

Though it's a 5-spoke ribbon spool of the same pattern as Underwood, the hub is different (and more complex, expensive to make).

This spool has a hub that is turned from solid material with an elaborate profile. The flanges are crimped/riveted onto this hub. One side of the hub is flattened and a small plate with two prongs is held in bearing-holes of the flanges. This little fork-plate can freely swivel around, presumably to lock the ribbon wound either way. It's unclear how this is supposed to work, very likely there should be a spring-clip to hold the swivel-fork in one of its positions.

As it now is, the construction does not hold a ribbon. How a spring-clip could be easily included to make it all work isn't clear (or I'm being dense here, of course a possibility ;-).

A brief browsing of a couple of patent databases didn't find any publication that could refer to this mechanism. Until the patent turns up to describe how it is supposed to work, it likely remains a bit of guesswork on what parts are missing (if any).

Even though the no-doubt clever mechanism now doesn't really work, it is a nice and shiny addition for any little 1920s portable :-)