Remington Model 11 Shotgun

[Dustman47]

I have seen two distinctly different barrel extensions on Remington Model 11 shotguns. I one, the left side of the extension is smoothly rounded it entire length. On the other there is a flat milled into that side that stops short of either end. Does anyone know if barrels with these extensions are interchangeable in a receiver. It is important so If you don't know, please don't speculate. I ask only authoritative answers to avoid a purchasing error.

[Thekarpetguy]

I just purchased a model 11. Serial number 2587XX. Can anyone help with a manf. date