The 31P battery is a commercial grade battery that appears to be about 3/4" larger than the group 27 in two dimensions. The 31P battery is the cross-reference for both the 2120 and 3415 on NAPA's and Batteries Plus websites. It just might be that the 31P will fit the 2120, but be too big for the 3415. Messicks lists the same battery (F170) for both tractors, but that could be because the smaller physical size will fit both. Of course, I don't know. It's just what I found with a bit of internet searching. Most cross-reference charts list the 2120 and not the 3415, so there may several false assumptions going on here.