Bentley showed two special edition models at Geneva this year to commemorate 60 years of hand-assembled beauty at the Crewe factory. The first ''Diamond Series'' car is a gussied-up Continental GT, slathered with an extra coat of luxury including unique 20-inch wheels, quilted leather, and special badging. Three new colors will be available. The second vehicle in the series is the Arnage, which gets the same cosmetic changes, but rides on 19-inch wheels. Engines are unchanged in both. Bentley plans to sell 400 Conti GTs and 60 Arnages, but has not yet announced pricing for either model.