OS stems are "Outside Stems" strips of wood bent around the outside profile of the boat. The rails or gunwales extending past the profile might indicate that there were originally OS Stems.
Regarding the profile, I was looking at the sweep of the bow & stern. Your boat does not seem to have the same amount of upsweep as the 1923 Charles River in the attached photo. Of course, models did change over the years, maybe someone else more familiar with the early years will jump in.
I misread Benson's post and thought he was giving you the build record for the boat you requested, but he was just giving you the closest numbers to yours. As Al mentioned, the early records are incomplete.
In any event, you have a very nice boat - keep plugging at it and keep participating in these forums, you will find a ton of expertise here.