26 USC 4053 - Exemptions

No tax shall be imposed by section 4051 on any of the following articles:
(1) Camper coaches bodies for self-propelled mobile homes 
Any article designed
(A) to be mounted or placed on automobile trucks, automobile truck chassis, or automobile chassis, and
(B) to be used primarily as living quarters or camping accommodations.
(2) Feed, seed, and fertilizer equipment 
Any body primarily designed

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(A) to process or prepare seed, feed, or fertilizer for use on farms,
(B) to haul feed, seed, or fertilizer to and on farms,
(C) to spread feed, seed, or fertilizer on farms,
(D) to load or unload feed, seed, or fertilizer on farms, or
(E) for any combination of the foregoing.
(3) House trailers 
Any house trailer.
(4) Ambulances, hearses, etc. 
Any ambulance, hearse, or combination ambulance-hearse.
(5) Concrete mixers 
Any article designed
(A) to be placed or mounted on an automobile truck chassis or truck trailer or semitrailer chassis, and
(B) to be used to process or prepare concrete.

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(6) Trash containers, etc. 
Any box, container, receptacle, bin or other similar article
(A) which is designed to be used as a trash container and is not designed for the transportation of freight other than trash, and
(B) which is not designed to be permanently mounted on or permanently affixed to an automobile truck chassis or body.
(7) Rail trailers and rail vans 
Any chassis or body of a trailer or semitrailer which is designed for use both as a highway vehicle and a railroad car. For purposes of the preceding sentence, piggy-back trailer or semitrailer shall not be treated as designed for use as a railroad car.
(8) Mobile machinery 
Any vehicle which consists of a chassis
(A) to which there has been permanently mounted (by welding, bolting, riveting, or other means) machinery or equipment to perform a construction, manufacturing, processing, farming, mining, drilling, timbering, or similar operation if the operation of the machinery or equipment is unrelated to transportation on or off the public highways,
(B) which has been specially designed to serve only as a mobile carriage and mount (and a power source, where applicable) for the particular machinery or equipment involved, whether or not such machinery or equipment is in operation, and
(C) which, by reason of such special design, could not, without substantial structural modification, be used as a component of a vehicle designed to perform a function of transporting any load other than that particular machinery or equipment or similar machinery or equipment requiring such a specially designed chassis.