J. R does indeed mean Jager Regiment or Infantry Regiment.
The "/" mark was not widely used in WW1 - certainly not part of the marking regulations.
It is however seen on post WW1 issue Reichswehr equipment based on my knowledge of swords and bayonets.
The other point is that the order of the marking for the Reichswehr is different to the Imperial method in that the first number is the sub-formation therefore I think your helmet belonged to the 13th company of the 22nd Jager Regiment.
Thanks for your help richie.
The helmet is a M35.