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I believe the entire handle, head, and connector bits. Probably also parts of the manufacturing process. When I bought a Mach 3 handle about 10 years ago there were at least 12 patent numbers printed on it.

There's a reason you haven't seen generic Mach 3 blades yet. The product came out in 1998, and blades from 1998 can still fit the system today -- no matter how many new patents Gillette chases, others will soon be able to reproduce exactly that.

https://www.google.com/patents/US6212777

Expires somewhere around 2018 (17 years after issuance)

I'd guess the attachment mechanism.