Fred, the Rock Island s/n 3576 has a SA 1910 dated barrel. Years ago I bought Rock Island 1903 serial number 64145 that had a RIA 3-07 dated barrel. The rear sight base was the open bottom and the rear sight leaf was also without the top rib and the slide also had the square left end like the sight pictured above. But both of these rear sight leafs have the Rock Island 7 which is different from the Springfield number 7 on the leaf scale