It looks 1970s to me. The paper on the back can be removed with no decrease to the value of the picture. You can easily apply another sheet of kraft paper and seal it after. To date it, the 5-digit Zip code system started in 1963 so it was framed after that. The date it was framed has almost nothing to do when the picture was painted. If you are looking for a S. Stephenson, I would start in Brunswick, Maine where it was framed.
As to value, I buy art because I like to look at it. If I do not like to look at it hanging on my wall, it ends up in a closet and I do not have any closets that are empty to hold pictures. If you like the picture and you like it on your wall, value is a distant concern in my opinion. My other opinion is just because it is an original does not mean it has value. Value comes with fame, name, demand, scarcity, condition, location, subject, and probably a host of other factors. JMO.
I do not like the Dollar Store unless I am looking for something that costs a dollar. I have bought
some good useable items at the Dollar Store for a $1 but when they start going $2, $3, etc., I can do better elsewhere.
Addenda: There is a Sandra Stephenson in South Freeport, ME that is about a 12 minute drive to Brunswick. She is 75. Maybe she is the painter?