For those that want to know the legal basis of the exterior "experimental" label and how it can be avoided with an "X" number, here you go (edited for relevancy and conciseness and marked for emphasis). Interestingly, I can't find a statutory basis for the interior passenger warning, but it's required nevertheless by FAA order:
FAR § 45.22 Exhibition, antique, and other aircraft: Special rules.
...
(b) A small U.S.-registered aircraft
built at least 30 years ago or a U.S.-registered aircraft for which an experimental certificate has been issued ... for operation as ... an amateur-built aircraft and
which has the same external configuration as an aircraft built at least 30 years ago may be operated without displaying marks in accordance with
§[45.23 (see below)] ... if:
(1) It displays ... marks
at least 2 inches high on each side of the fuselage or vertical tail surface consisting of the Roman
capital letter “N” followed by:
...
(ii)
The symbol appropriate to the airworthiness certificate of the aircraft (“C”, standard; “R”, restricted; “L”, limited; or
“X”, experimental)
followed by the U.S. registration number of the aircraft....
FAR § 45.23 Display of marks; general.
...
(b) When marks include
only the Roman capital letter “N” [i.e., with no "symbol appropriate to the airworthiness certificate of the aircraft" per 45.22(b)(1)(ii) above] and the registration number is displayed on ... experimental ... aircraft,
the operator must also display on that aircraft near each entrance to the cabin, cockpit, or pilot station, in letters not less than 2 inches nor more than 6 inches high,
the word[] “experimental,” ....