It appears to be a selector in the left trigger slot. That is the same layout as a Parker Bros. single selective trigger, but that selector slide looks different than I'm use to seeing on Parkers. Others are likely more familiar with these single trigger variations than I.
Parker Bros. Single Selective Trigger 1923 01.jpg
Parker Bros. Single Selective Trigger 1923 02.jpg
I'm guessing if you slide it forward it will cover the L and an R will appear at the back end of it. Then your trigger will shoot the right barrel first.
What is the vintage of your gun? There were a number of after market single triggers being offered in the years before Parker Bros. offered one.
Threaded Mode