6. All receptacles containing ¹[class I] petroleum shall have a stamped, embossed, painted or printed warning, either on the receptacles itself or, where that is impracticable, displayed near the receptacle, exhibiting in conspicuous characters the words "Petrol" or "Motor Spirit" or an equivalent warning of the ²[class I] nature of the petroleum: Provided that this section shall not apply to- (a) any securely stoppered glass, stoneware or metal receptacle of less than ³[nine litres] capacity containing ⁴[class I] petroleum which is not for sale, or (b) a tank incorporated in a motor conveyance, or attached to an internal combustion engine, and containing intended to be used to generate motive power for the motor conveyance or engine, or (c) a pipe-line for the transport of petroleum, or (d) any tank which is wholly underground, (e) any class of receptacles which the Government may, by notification in the official Gazette, exempt from the operation of this section.