Antique mystery scale weight? Or depth finder?

[janem2017]

I am curious if anyone knows what this was used for? It appears to be a hanging scale slotted weight but I have yet to find anything comparable online. It weighs 43 pounds and is over 100 years old. The only marking left are a -3W which makes me wonder if the dash is a 4 or if that's a coincidence. 43W=43 pounds. This has been a mystery for 2 generations, we know that it was considered to be special by our Grandfather but not why.. there is a memory as a child where an explanation included a rope and piano wire. Any suggestions or guidance is greatly appreciated!