Minor series designations for different denominations of notes do not always change at the same time. This can result in what appear to be the same series, but of different denominations, of notes not having the same signatures. The various series 1969 notes are an example of this.
In 1969, the $1, $5, $10, $20, $50 and $100 (no $2 were printed) denominations all changed with a major series change to 1969 series with the new US Treasurer and Secretary of the Treasury Elston/Kennedy signatures.
When Elston married and changed her name to Kabis a minor change occurred and the $1 1969A series was created, but there were apparently sufficient supplies of 1969 series notes of the other denominations available and those denominations did not change to 1969A series at that time.
When Connally replaced Kennedy as the Secretary of the Treasury another minor change occurred and this generated the $1 1969B series. Notes were now also needed for the other denominations and the 1969A series for the other denominations was created. All notes now had Kabis/Connally signatures, but at this point the series suffixes got out of sync.
When Banuelos replaced Kabis as US Treasurer only 5 months after Connally took office for yet another minor change, the $1 1969C series was created and the 1969B series was created for the other denominations (except $100s, as none were needed). All notes now had Banuelos/Connally signatures, but the series suffixes did not sync up.
When Schultz replaced Connally as the Secretary of the Treasury for the last minor change in the 1969 series, the $1 1969D series was created and the 1969C series was created for the other denominations, again except for $100 note. The odd thing is that the 1969B series designation for the $100 was skipped. It was given a 1969C series designation (your guess is as good as mine as to why). All notes now had Banuelos/Schultz signatures, but the series suffixes were still not in sync.
Finally, when Simon became Secretary of the Treasury in May 1974 and Neff US Treasurer just a month later in June 1974, a major series change occurred and the 1974 series was created. All denominations series were now once again in sync with Neff/Simon signatures.
Sort of a long answer to a short question. Hope it helped