I have a "Very Late" H grade Lefever with one of these Infallible triggers on it. The button is the selector. They have the unusual feature of the selector itself moving everytime gun is fired & it does not reset itself upon opening the gun. Thus if you normally used one for hunting with it set for right bbl first & fired only one shot & reloaded, you would have to set back to right or would fire left first next time around. I think there may be a possibility that some of these triggers may have been fitted at the Ithaca plant on late guns, but have nothing positive on this. This ia an earlier gun & would undoubtably be an after market instlation. Mine has 1912 patent dates on it which are by A H Worrest of Lancaster PA.
