Not true for a long time - see 35 USC 114 "Models, specimens" including "The Director may require the applicant to furnish a model of convenient size to exhibit advantageously the several parts of his invention."
This rule was implemented in 37 CFR 1.91: (a) A model or exhibit will not be admitted as part of the record of an application unless it:
(1) Substantially conforms to the requirements of § 1.52 or § 1.84;
(2) Is specifically required by the Office; or
(3) Is filed with a petition under this section including:
(i) The fee set forth in § 1.17(h); and
(ii) An explanation of why entry of the model or exhibit in the file record is necessary to demonstrate patentability.
no subject
This rule was implemented in 37 CFR 1.91:
(a) A model or exhibit will not be admitted as part of the record of an application unless it:
(1) Substantially conforms to the requirements of § 1.52 or § 1.84;
(2) Is specifically required by the Office; or
(3) Is filed with a petition under this section including:
(i) The fee set forth in § 1.17(h); and
(ii) An explanation of why entry of the model or exhibit in the file record is necessary to demonstrate patentability.