The buckle supplied was actually plated, rather than 18K gold. These were initially sold as just the head, and you had to supply your own strap but later on they were sold with either the strap or bracelet ex-factory.

I tried to photograph my buckle, but it seems the macro mode on my compact is more a close focus priority mode than true macro, so it didn't come out all that well:

You'll note the classic sun and "SWISS" text, and also the "PLAQUE G" denoting that it's gold-plated. These elements seemed to shift position over time, as you can see from Gianvittorio's photo of the buckle of his later watch (~20000 serial numbers later than mine):