I will be putting a set of shackles on a 21 touring tomorrow and would like to know if the shackles go in from the rear or front?
Sounds like you are installing the "U" shaped ones, I installed: nuts to the rear for front and same on the rear ones. The "U" shaped ones are replacement type anyway so guess which ever way you like. 21 would have had the "L" ones.
Scroll down to the bottom of this thread, it shows the "L" shaped spring shackles that Mark G. is talking about: