Frank you have one of the early Miller triggers. I have one just like yours on a DHE 20 mfg in 1927. The trigger works in two ways, push it forward in the normal mannner and the right bbl is fired first, push it back and the left bbl fires first hence the RSL mrkings. The middell position is safe.
I have had my gun about 6years and have had no problems. I have other Parkers with a Miller trigger as well and all function flawlessly.
You have my "Holy Grail" of Parkers the 410 congrats
