Can't help with info on the pins, but can give you the manufacture dates of both you and your friends scatter guns. Yours was made in March of 1983, while his was made in September of 1984.
This was basically the dawn of the end of the Stevens 311 side-by-sides and associated variants, as they would only remain in production for a few more years. My guess is that the difference in pin locations has to do with design changes over the years to lower production costs and/or require less hand-fitting during assembly.
Here's the page from the 1983 catalog detailing the SxS models offered that year.
https://www.savageshooters.com/attac...6&d=1425912817
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