The pad you have wiped clean should have a combination of numbers, possibly separated by an "X".
Since your car was built on April 1st, I would expect a casting date of 2x or 3x for February or March casting.
Regardless, something that would indicate prior to April 1968.
Seems like factories didn't care about readability on these 2bbls, almost everyone I looked at for examples last night were barely readable.