You need to look at the full hallmark, not just the date letter, then you will see that there is a distinct difference between the two date letter cycles.
It may be disloyal of me, but the British date letter charts on our site
here are not particularly good. They only show the date letters, omitting the rest of the hallmark; they leave out most 20th century dates; and there is at least one error (in the Birmingham chart).
Phil