This isn't the first time I've seen something like this, but it's the most recent.
This is a link to a popular card dealer's YouTube channel. Every few weeks he highlights the top sellers at prominent auctions around the country.
At about the 7 minute mark he showcases a 1966 unopened box which sold recently. If you watch for a few seconds, you'll see the box has a BBCE seal on the bottom that says it's a "series 2/3 box".
Were there boxes with mixed series packs? I'd seen a couple other boxes before like this - all from some year before 1973, and whether they were all BBCE sealed or not I can't say, but they all carried a designation indicating they were from 2 consecutive series.
Thoughts?
https://www.youtube.com/watch?v=PiIYKy5FIXc