My memory is not what it once was, but IIRC that revolver uses the flat mainspring common to revolvers of that era. If so, it is a sort of shallow U with the tip of the longer part fitting into a notch in the bottom of the hammer and the shorter end going into a notch in the bottom of the grip.
Installing the mainspring is just a matter of bending it to fit both ends where they need to go.
If that is not correct, please post a picture of the gun with the grips off and the spring you are trying to install and maybe we can help.
Jim