The 5100 came before the 311.
The 311 in 1940/41 I think.
Same action type but there were some changes over the years. It may account for the change in hammer & sear pin placement.
To confuse things, guns were marked with either or sometimes both designations (311 / 5100) for some time into the early 50's.
The '101.7' thing is a Sears Roebuck model designation and will be found on the guns with Stevens markings as well as the off brands. You'll see JC Higgins marked guns with the designation.
There is another early Stevens SxS that looks quite alike to the 311/5100 gun, but inside it is coil spring-striker fired, not hammer.
I don't recall what the Model number was on those. Maybe they called them 5100's too!, but I doubt it. That could account for pins being in a different place..
Lots of stampings and sheet metal formed parts inside those and can be a pain to work on.
Stevens markings, model numbers are very confusing. Just when you feel you may have it figured out, you usually don't.
Stevens(Savage actually) started using 'date codes' on their guns in 1949 and used it till 1970. Since most of the shotguns weren't serial numbered (didn't have to be by law till '68), guns in that era can sometimes be dated by the code.
http://www.wisnersinc.com/additional_info/stevens_date_code.htm