Found it!
Section 432.2 Handguns, what I read in subsection A is that if you are shipping a handgun that meets the definition of Antique can be shipped between C&R holders (I didn't know that), but it also says in subsection B if the handgun meets the criteria for Curio and Relic it may be shipped without restriction.
THAT GIVES ME PAUSE, as any gun that is over 50 years old is a C&R. the part I am wondering about is which are certified by the curator of a municipal, state, or federal museum that exhibits firearms to be curios or relics of museum interest,
I read that that you have to have a "certification" on a specific gun, maybe?
432.2 Handguns
Handguns and other firearms capable of being concealed on the person are nonmailable unless mailed between the parties listed in this section, after the filing of an affidavit or statement described in 432.22 or 432.24, and are subject to the following:
Firearms meeting the definition of a handgun under 431.2 and the definition of curios or relics under 27 CFR 478.11 may be mailed between curio and relic collectors only when those firearms also meet the definition of an antique firearm under 431.3.
Firearms meeting the definition of a handgun under 431.2, which are certified by the curator of a municipal, state, or federal museum that exhibits firearms to be curios or relics of museum interest, may be accepted for mailing without regard to the restrictions provided for handguns in Exhibit 432.25 and in 432.21 through 432.24.
Air guns that do not fall within the definition of firearms under 431.1 that are capable of being concealed on a person are mailable, but must include adult signature service under DMM 503.9.0. Mailers must comply with all applicable state and local regulations.
Parts of handguns are mailable, except for handgun frames, receivers or other parts or components regulated under Chapter 44, Title 18, U.S.C.
Mailers are also subject to applicable restrictions by governments of a state, territory, or district.