The serial says S- and 1988 Pine mountain in one post was marked as "AS", so the A should be hidden .
More about serial numbers in:
https://www.retrobike.co.uk/threads/decoding-marin-frame-numbers.433668/
https://www.retrobike.co.uk/threads/help-with-marin-frame-numbers-please.37430/
And here is one frame like yours:
https://www.retrobike.co.uk/threads/1988-marin-pine-mountain-restoration.409079/
More about serial numbers in:
https://www.retrobike.co.uk/threads/decoding-marin-frame-numbers.433668/
https://www.retrobike.co.uk/threads/help-with-marin-frame-numbers-please.37430/
And here is one frame like yours:
https://www.retrobike.co.uk/threads/1988-marin-pine-mountain-restoration.409079/