On the first two attempts, I placed the weak spring under the trigger mainspring. I filed out a channel in the trigger plate for it to fit in. This weakens the plate and caused it to bend in this area when the mainspring was tightened.
On the third attempt, I placed the weak spring on top of the mainspring, dropped the channel and thicken the plate in this area (5mm).
I made a plywood mockup of the pistol stock to test the trigger. I copied the lock bolt placement from the stock to the mockup to get the lock placed correctly. This trigger works like a dream and fires the lock perfectly.
I also found out a few interesting things:
The trigger would not fire the lock if the triggerbar is almost touching the sear. I had to file down the trigger bar to get about a 1/8 gap between it and the sear. Apparently it needs this space to pick up enough speed to trip the sear.
The distance from the top of the back lash screw to the underside of the mainspring it a little less than a 1/16 when the trigger set. The spring hardly moves at all when fired.
I am going to tweak the mainspring a bit more and see how far I can reduce the curve. If I can reduce the curve, I can reduce the depth of the trigger mortise. I am also going to cut off about ½ from each end of the trigger plate.
Best regards
Rolf