Off-highway vehicle exception focuses on design, not use

A customer recently inquired about buying three of our completed vehicles. When we sell this type of vehicle, we typically charge Section 4051 tax on the chassis and body. However, in this case the customer is willing to certify that the vehicles will be used exclusively at job sites and that they will not travel over public roads. Does this type of certificate qualify the vehicle as a nontaxable off-highway vehicle?