Here is an example of a DT conversion that I did on a DHE 12g.
The original framework of the SST was gutted and reused. This was so that the original safety link toggle could be used. The safety toggle needed a block welded onto the end of it to reach the location of the new triggers. The framework also had to be modified in order to install a trigger return spring.
The frame was permanently attached to the trigger plate with the old screw and some tack welds.
The outside of the plate was then modified to remove all evidence of the SST ever having been there. L&R filled in, the long trigger slot filled as well as the two screw holes in the rear.
This frame will be color cased, so the surface work to the trigger pate is of no concern.
The other thing to consider is the stock dimensions as well with a DT conversion. Going to a DT will move the trigger finger forward. This particular gun had it's LOP effected by nearly 5/8".
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