That looks like the California SB100 "approval" plate...it is a way to register an "assembled (replica) car" like GT40s. It can be important because as I understand it once a car has SB100 registration in California it is exempt from smog testing and can be run on the roads like any other car.
It is not the chassis number (at least, I don't think so...but I don't live in CA so I have no idea if it is or not...might be!) SB100 approvals might well have been numbered in series, though, so hopefully others will chime in and help clear up my confusion.
The plate is riveted to the chassis. There are also numbers (build number, NOT chassis number) bonded into the removable fiberglass panels. The build number is also marked or etched into some of the steel panels. I am not aware of a chassis number stamped directly into a GT40 tub. It could be done at the factory on request.