1. This adapter fills the void in the NM grip frame behind the trigger. It has a hole in the left end the size and depth of the return spring and plunger hole in your OM grip frame. The small hole in the side is for retaining it in the NM grip frame using the existing NM trigger return spring retaining pin. The depression on top is for clearance of the hammer base.
You can make the simple aluminum adapter above to fit in the NM GF behind the old model trigger or just use a .22 LR case to hold the return spring and plunger. Either will need the one NM trigger return spring retaining pin closest to the trigger shown below to hold your OM trigger return spring/plunger adapter or .22 case in place. File a small notch in the .22 case head to nest on the retaining pin.
2. Another option is to use the NM trigger return spring with a Clements adapter trigger. This is the easiest way and the trigger is wide to fit the wide trigger slot in all NM grip frames. You'll need the NM grip frame trigger return spring and both pins to use a Clements trigger.
___________________________Clements adapter triggers are the center two_________________________
OM trigger__________________standard____&____Bisley w/over travel stop__________________NM Bisley
They are $65 and are a 'drop in'. http://www.clementscustomguns.com/