Are you sure that "Y" isn't an "X"?
The Y prefix was European, and was made between 3/11/54 and 6/27/74. According to my source, there were only two of those exported to the US (both reportedly in 1956), and neither is your serial number.
Perhaps the man was in the armed services and brought a gun back with him from Europe?
Keep in mind, that prefix doesn't make it any more rare or valuable, just means that it's a gun that wasn't built for the US market.
Now, what doesn't make sense about that is that you indicate it's a Browning. John Browning designed the gun, but Browning hasn't ever built any shotguns, instead they have farmed it out to other companies. A gun built on that patent for the European market wouldn't say "Browning" anywhere on the gun (other than maybe something like "Browning's patent"). It should say "FN" or "Fabrique Nationale" who was the company that actually built the Belgian made run of Brownings for Browning.
Perhaps one of the A5 experts will be along with more information...