My suspicion is that it was not used for the 350.
In 1967, the 327 and 350 had different part numbers - both for short block engine assemblies and “fitted blocks”. A fitted block in generally a short block - minus the rods and crankshaft.
The CE coding did not start until September of 1967. Prior to that, only complete engine assemblies revived a stamped code on the pad. Short blocks prior to September 1967 had blank pads.
Now you have me curious and probably won’t sleep much until I find an answer.