A handy person from Jerseyville, IL says:
May 29, 2016
I believe this is a block and tackle spring balancer. I have 4 windows, each with 2 of these. No numbers, no stamps. I'm not even sure what brand of window it is because I can not locate any identifiers. One on each window has broken so I'd like to replace all of them.
Paul from SWISCO responded:
June 1, 2016
Please see our Series 860 balance, I think this could be what you're looking for.