It's a threaded cap with a copper washer. I had one fall out of my 748 when it only had a few hundred miles on it. I'm sure they are probably the same among models. Here are the part numbers from the 2002 748 parts manual...
Cap - 780.1.007.1A
Washer - 420.2.001.0A
According to the parts manual, when you order the cap the washer comes with it, but you can order the washer separately.