26 C.F.R. § 48.4063-1
Under the provisions of section 4063(b), the tax imposed by section 4061(a) shall not apply to bodies sold by the manufacturer thereof to a manufacturer (but not an importer) of automobile trucks (as defined by § 48.4061(a)-3(a) ) to be sold by the purchaser. Thus, a manufacturer of automobile truck bodies is permitted to sell such bodies tax free to manufacturers of automobile truck chassis. This section does not apply with respect to the sale of an automobile truck chassis to manufacturers of automobile truck bodies. However, see § 48.4061(a)-1(e) with respect to the sale of an automobile truck chassis for use in the manufacture or assembly of a nonhighway vehicle (within the meaning of § 48.4061(a)-1(d) ). In order to effect a tax-free sale of a body as provided in this section, both the seller and purchaser must comply with the registration and other requirements of section 4222 and the regulations thereunder. A chassis manufacturer who purchases a body tax free as provided in this section shall, for purposes of application of the tax imposed by section 4061(a), be considered the manufacturer of such body.
26 C.F.R. §48.4063-1