My FIL gave me a NIB balun he has had for eons. The paperwork for it says it is broadband from 3 - 40 MHz. Obviously this is from the days prior privileges on 160M. Is there any reason I couldn't use this on 160M?

Might be a bad match at those lower frequencies.  Gets lossy and hot.  Use a lot of of your  RF heating up the balun instead of the ionosphere.  Does the paperwork have any graphs of loss versus frequency?

Pav

No info about losses at various frequencies. Time to hit the online radio shops to find a new balun. Thanks!

It probably just doesn't have enough "balun" to work on 160m, it's a common design compromise.  160m would require twice as many ferrites, so the balun would be twice as long and quite a bit more expensive.  160 isn't a big deal to most amateurs because of the antenna size, noise and propagation, and for those who are active verticals are a lot more common due to the propagation characteristics.  If it's the typical ferrite bead style balun, you could always add a few additional ferrites, or a small coil of coax as a feedline choke.

Or just ignore it, and/or ground the feedline shield at some point before it goes in the shack.

It likely would not predate the authorization of amateur radio frequencies at 160 meters because hams have used that band essentially "forever"...