A pre 1988 aircraft does need to have the aircraft type and serial number
visible on the exterior of the aircraft. It does not have to be in the form of
a plate but the data has to be there.

FAR 45-13
(d) On aircraft manufactured before March 7, 1988, the identification plate
required by paragraph (a) of this section may be secured at an accessible
exterior or interior location near an entrance, if the model designation and
builder's serial number are also displayed on the aircraft fuselage exterior.
The model designation and builder's serial number must be legible to a person
on the ground and must be located either adjacent to and aft of the rear-most
entrance door or on the fuselage near the tail surfaces. The model designation
and builder's serial number must be displayed in such a manner that they are
not likely to be defaced or removed during normal service.

John Dupre

Having said that, any aircraft with a legal manufacturer's data plate on the
aircraft does NOT NEED a second data plate back by the tailfeathers. I just
reread part 45, and nowhere does it demand a second data plate. Your FSDO
may
be exerting their fifedom above and beyond the law, but there is no FAA
requirement for the second data plate.

Jim
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor