World War One Nerd et al, this may be of interest it's lifted verbatim from the "Manual of the Motor Transport Corps", October 1918.
513. IDENTIFICATION INSIGNIA.
(a) Trucks composing trains or smaller units of not less than twelve trucks
will have stencilled or painted on both sides and the rear distinctive insignia,
together with the serial number, of such truck within the company or other unit.
The design must be of simple pattern, readily discernable at some distance, and
must be black, but on a background of any color, which must be more than
eighteen inches square. The same design and the same colored background will
be used for all vehicles in a company or smaller unit, while the same design,
but of different colored background, for each company will be used for thevehicles
in a train. Motor ambulances and passenger cars will also bear such
insignia, so far as body construction will permit.
(b The organization number will be placed within the square containing
the insignia in such a manner as not to create a tendency to confuse it with the
registration number, and will indicate the vehicle's number within the organization.
Thus, if an organization operate twenty-seven trucks or ambulances and
one passenger vehicle, the trucks or ambulances will be numbered from one to
twenty-seven consecutively, and the passenger vehicle twenty-eight.
(c No insignia will be used or adopted for this purpose until after a design
or description thereof shall have first been submitted to the Director, Motor
Transport Corps, and his approval given. Approved designs will be registered
in the Office of the Director to prevent duplication. In cases where organizations
find it impracticable to cut their own stencils, requisitions therefor should
be submitted to the Director, Motor Transport Corps.
(d) The provisions of the foregoing Paragraph 513 (a) are limited to the
Train headquarters and Military Police.
Mobile ordnance repair shop.
Corps headquarters (including headquarters troop).
Military police company.
Corps artillery park (including M. O. R. S.).
Troop transportation trains.
Engineer regiment and engineer train.
Army headquarters (less army artillery headquarters).
Army artillery park.
Military police company.
Motor supply service (1 regiment headquarters and 6 companies engineers).
Truck companies, army train.
S. O. S.
Motor truck trains.
Motor truck companies.
Motor car companies.
In addition to the foregoing, various organizations operating in the Services
of Supplies, such as groups, schools and training centers, may, upon application
to the Director, Motor Transport Corps, be assigned identification insignia
where circumstances warrant such action.
(e) Motorcycles, rolling kitchens, trailers and vehicles belonging to combat
trains of organizations will not be marked with identification insignia.