37 CFR §1.91

1. (a)A model or exhibit will not be admitted as part of the record of an application unless it:
   1. (1)Substantially conforms to the requirements of § 1.52 or § 1.84;
   2. (2)Is specifically required by the Office; or
   3. (3)Is filed with a petition under this section including:
      1. (i)The fee set forth in § 1.17(h); and
      2. (ii)An explanation of why entry of the model or exhibit in the file record is necessary to demonstrate patentability.
2. (b)Notwithstanding the provisions of paragraph (a) of this section, a model, working model, or other physical exhibit may be required by the Office if deemed necessary for any purpose in examination of the application.
3. (c)Unless the model or exhibit substantially conforms to the requirements of § 1.52 or § 1.84 under paragraph (a)(1) of this section, it must be accompanied by photographs that show multiple views of the material features of the model or exhibit and that substantially conform to the requirements of § 1.84.