This was discussed here....
And I found this
UNITED STATES OF
FEDERAL AVIATION AGENCY
Civil Air Regulations Amendment 1-6
Effective: July 25, 1963
Issued: July 18, 1963
[Reg. Docket No. 1864; Amdt. 1-6; Supp. 1]
PART 1-CERTIFICATION, IDENTIFICATION, AND
Exceptions To Marking Requirements
which partway down says........
Currently effective §§ 1.102(a) and 1.103(a), which became effective on December 31, 1960, prescribe 12-inch identification marks, to be located either on the side of the fuselage, or on the vertical tail surfaces, for fixed-wing aircraft. Compliance with this provision introduces the anachronism that operators of antique aircraft find objectionable. In general, they wish to display, instead, the formerly prescribed 20-inch wing marks and 2-inch side fuselage or vertical tail surface marks.
The Agency adopted the 12-inch side identification marks as standard for fixed-wing aircraft as a means of decreasing the collision hazard associated with air-to-air identification of civil aircraft by U.S. Air Force interceptor aircraft engaged in national defense. In addition, the Agency’s air traffic controllers had advised that such marks aided in the control of air traffic by facilitating the identification of aircraft.
More recently the Agency has been informed by the U.S. Air Force North American Air Defense Command that it would have no objection to the deletion of the requirement for side fuselage or tail markings on antique aircraft which are operated at less than 180 knots TAS
within the continental limits of the United States, except for the Florida area, but that it would object to granting similar relief, under similar conditions, for all nonantique aircraft. Relevant also is a new rule, recently adopted by the Agency as part of Amendment 60-24 effective December 26, 1961, which requires that aircraft operated to, from, or on an airport at which an airport traffic control tower is operated by the United States Government be capable of two-way radio communication with that control tower. With two-way communication available, control tower personnel now have little need to visually identify aircraft by means of its identification marks.