My truck's ID code is H55 and the lower grill panel is chromed as are the parking lamp panels. Importantly, this truck is not a deluxe cab which leads me to believe that the chrome option was available on some non 3104 trucks or could be ordered. As a result of my inspections of the truck and my discussions with the PO I am convinced (as I can be with a 50 year old truck) that the chrome grill is original.
In my search for a first series 55 AD truck, I only found two other trucks both in too poor condition to purchase. One was advertised as a 54 but it was a 55.1 and the door plate showed H55. The second truck was marked the same (H55). Both of those trucks had painted grills but unfortunately the original paint was gone or overpainted in both cases. I believe that the white with black background was the factory paint on non-chrome grills as you state.
I suspect that the Chevrolet on the upper bar of my grill was originally painted black but the paint is gone. I say this as the chrome side badging (55 badges) had a black background and some of the original paint remains.
I agree that the grill was an obvious pointer to the difference between the two model years, The other being the side badging which is plainly different in 55.
I saw your website over at Kookbolt--your truck, the carryall and their restorations are impressive