Are you sure this is a 27" rim? I thought there was no such thing as 27" tubulars and that they are all really 700 (622mm). Just an issue if you were thinking about replacing the rim or wheel.
Schwinn often referred to the way they built wheels as "tubular". They took a piece of tubing and then formed it into rim. They are a clincher wheel.
For the OP, S-6 was a rim size on 24"&26" rims and a Schwinn specific size. Schwinn's 27" rims are the same size as other manufactures.