In plain English
FAR 52.208-6, Marking of Leased Vehicles, is a contract clause prescribed at FAR 8.110, most recently dated Apr 1984. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 8.1104(c), insert the following clause in solicitations and contracts for leasing motor vehicles, unless the motor vehicles are leased in foreign countries:
Prescribing reference: FAR 8.110.
The official text, verbatim
As prescribed in 8.1104(c), insert the following clause in solicitations and contracts for leasing motor vehicles, unless the motor vehicles are leased in foreign countries:
Marking of Leased Vehicles (Apr 1984)
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(a) The Government may place nonpermanent markings or decals, identifying the using agency, on each side, and on the front and rear bumpers, of any motor vehicle leased under this contract. The Government shall use markings or decals that are removable without damage to the vehicle.
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(b) The Contractor may use placards for temporary identification of vehicles except that the placards may not contain any references to the Contractor that may be construed as advertising or endorsement by the Government of the Contractor.
(End of clause)