The cutoff date for original 2” numbers is around 1982, give or take:
§ 45.29(B)(1)
(i) An aircraft displaying marks at least 2 inches high before November 1, 1981 and an aircraft manufactured after November 2, 1981, but before January 1, 1983, may display those marks until the aircraft is repainted or the marks are repainted, restored, or changed;
Yes, that's true:
§45.29 Size of marks.
(1) Fixed-wing aircraft, must be at least 12 inches high, except that:
(i) An aircraft displaying marks at least 2 inches high before November 1, 1981 and an aircraft manufactured after November 2, 1981, but before January 1, 1983, may display those marks until the aircraft is repainted or the marks are repainted, restored, or changed;
BUT, further down it says this:
(iv) Marks may be displayed on an exhibition, antique, or other aircraft in accordance with §45.22.
and that says:
§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 under §21.191(d) or 21.191(g) for operation as an exhibition aircraft or 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 §§45.21 and 45.23 through 45.33 if:
(1) It displays in accordance with §45.21(c) 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:
(i) The U.S. registration number of the aircraft; or
All Cessna 150s and 152s are more than 30 years old, and they're small, so I guess we can all do 2 inch numbers.