When if ever did Remington outfit the 510P with a barrel mounted rear sight that was mounted in a dovetail slot? All the early 51x series gun that I've seen have barrel mounted rear sights attached with 2 screws, never via a dovetail in the barrel. In the past few days, I seen 3 guns like this (dovetail mounted rear sights) in passing. Wish I could have had a closer look and got the date code on thebarrel.
