This one is a real quandary. I would really like to know the real answer, but it id difficult to pin it down. We've had counterweights in theater at least as far back as 500 BC and the Greek's
Deus ex machina. But to pin a date on "as we know it today" becomes a
bit more difficult.
Counterweight rigging can be specifically traced to 1895 but I can't determine if it is the two rod type of
arbor we use today or not.
An interesting side note.
Clancy listed a "
Counterweight Holder" in it's 1890 catalog. Clearly not the
counterweight arbor we know and "love" today, but a start.
Another interesting note, is the sheet metal walled
arbor Clancy offered in 1905. It obviously did not sell but now in 2011, the Thern Brickhouse
system Thern Stage Equipment | Counterweight Systems
is reintroducing the concept. The Thern version solves the problems with the early
Clancy product with the cam-over locking rod and such and the ergonomics of no-tilt front loading arbors without the need of spreader plates is a major improvement. What's old is new again!
The research I have available shows the two rod
arbor was in place in 1921, but as we know, things don't just pop into existence. How long before the catalog evidence did the product actually get installed and used for the very first time? I don't really know, but if anyone comes up with better answers, I'd love to know.