Prestone stencil?

[deadline]

I'm seeing CCKWs (restored) with PRESTONE 41 or 44 on the hood, just above the grill.

I even have a picture of two CCKWs, from the SAME UNIT 666th QM Truck Co. (page 33 of the TankoGrad book).. the CCKW on the left does NOT have the 'Prestone' stencil.. the one on the right DOES! The left truck is an early truck, the right a later one.

Any ideas on when the stencil started use?