Thanks everyone
Yes Jeff - I have thrown away a number of decals - i had to hunt this one down.
From MOCA detailing concours guidelines - "1967 Air cleaner Crankcase vent decal C1AE-6A720-A"
My best guess would be the breather cap decal considering the part number starting with C1AE
So it would be placed on the top surface of the breather cap. The challenge IMHO is that IMHO these were not originally used but you have to make a choice between original and how the organization expects things to be.
Base of the requirement is likely older (or possibly current) MCA rules that were shared with MOCA