From the Einstein product page:
Quote:
A frosted pyrex dome protects the lamp and tube from damage and serves to reduce UV emission from the flashtube. It equalizes the effective size and shape of the modeling lamp and flashtube as the modeling lamp filament is in exactly the same focal point as the UV-coated flashtube, resulting in an extremely close relationship between the flash and modeling light. The dome lowers the color temperature by approx. 300K. An optional UV-coated dome (sold separately) further lowers the color temperature by approximately 600K.
It sounds like part of the design and that Buff service forgot to return it to you. I'm guess that it also protects in case the modeling lamp bursts (as high power halogen bulbs sometimes do when they fail).