The 40's insert is a narrower version of the 70's, designed for the Snareweight JR (amazing with the #5 Snareweight as well if looking for less dampening than the 70's). 
They respond to harder hits with some intentional movement, making it ideal for drummers that want variation while playing. They also tend to give the snare the MOST possible high end presence before going darker sounding. 
 
The 40's will add to your existing JR setup live as well, giving you more colors in a snap for tweaking the snare on the fly.