I have a Browning humpback Sweet Sixteen with the markings Y PV that are stamped vertically on the receiver. Any idea what these mean. I know the choke is mod based on the marking on the barrel?

What you are looking is part of Belgium's proof markings system. The PV stands for "Poudre Vive" (smokeless powder). The "Y" I cannot explane, as that character does not appear in any of my references on Belgium proof marks. FN did make a "Y" series of Auto 5s but none of them were supposed to have been imported into this country and that "Y" desigation should have been part of the s/n on the underside of the reciever. You should take another look at this "Y" an insure that it is indeed what is stamped there.