If you reproduce it, my suggestion is that you replace the hammer spring (currently a leaf spring which applies more force to the hammer as it is cocked) with a coil compression spring. If you mount it correctly, you can have the spring pointing JUST above the pivot point of the hammer when the rifle is fully cocked. This "variable geometry" results in very little load being placed on the sear/hammer engagement and the resulting trigger will be almost dreamlike ... I've never seen a Mod 6 Improved, but I have often wondered if that improvement was what they made.
Also, be very careful NOT to cut any sharp/square internal corners on the breech block ... if you are using a mill it can be tempting/quick to do, but will result in HUGE stress concentrations and a failing breech block is NOT a good thing in this design.
Oh yeah ... one last idea ... quick swap barrels for both .22Mag and 17HRM are totally viable with this design ...