When they switched from nitro to poly. Don't know the year but if you spray nitro on a decal the solvent would eat it...
Nineteen sixty eight was when they started clearing in "poly." But the headstocks were clear-coated in nitro for another year or two.
Year, or so? The front of the headstocks on basses were clear-coated in nitro until the mid 70's. That's why the front of those look really "orange" compared to the rest of the neck.
By early '74 they were clearing with poly. When was the exact date Fender switched from aliphatic urethane to poly? Not easily found. The orange tint is because nitro has a yellow cast to it. Poly's are blue. That's also the reason nitro looks "warm."
I had a '73 P bass back in '73. The headstock face was originally very clear, like the poly on the remainder of the neck. The yellowing on the headstock face developed over time. I wasn't aware of the different (nitro) finish on the face; I thought the yellowing was caused by the stage lights striking the face more than the rest of the neck. -
From what I understand, the decals that Fender had when they used nitro, were made for nitro. When Fender went to poly, they discovered that it would shrivel the nitro decals up into a little glob. Since Fender had a... well, a lot... of those decals; and, never being ones to waste anything, they simply continued to use them - and finished the front of the headstock in nitro, until the old decals were used up. Then they started using "poly decals"...