A little late to the party but going by the Ford parts books:
1966 individual year published January 1966 shows the same C5ZZ-6527886-A part number as the 65 book does.
1967 individual year published January 1967 shows the new C5ZZ-6527886-B part number with the dent that was serviced by Ford into the 1990's.
Since these books took a while to research, write and paste up for print the actual information is probably correct back to September/October or even earlier.
1960-1968 Combined year fiche published June 1968 shows 65-66 with the "A" version and 67-68 with the "B" version.
Somewhere buried in this or the old Vintage forum was a discussion that surmised the changeover was sometime in 1966.
In the unrestored 1966 threads:
Late SJ 6R07C23137X showing the dent. Estimated date by the Production guide would be around 15G
Late NJ 6T07C3669XX, 18F date with the dent.
Early MI 6F07T1272XX, 10/1/65 date without the dent.
It seems that the change happened sometime in 1966.