Wow
An ancient History question.
More to what John said

The first open edition was Cape Hatteras and was produced in China.
The manufacturer was told by Harbour Lights that each year was going to have the next letter of the alphabet. So for the Hatteras, the prefix for the 1995 issue was going to be the letter “B”.
Therefore, when the Stamp Series came out in 95 the manufacturer added the “ B” prefix to the first 331 thinking that “B” stood for the year not for the production year. When the mistake was found, the letter for the rest of the year was changed to “A”
Then 1996 came and the manufacturer decided to start the first 331 with the letter “A” to fill the void that was created by their lettering mistake the year before. The plan was to then switch to the“B”prefix after the 331 but then an other issue came about. The collectors demanded that Harbour Lights Didn’t create an open edition until the L.E. was made first. The “B” prefix was then taken off for the Spectacle Reef, St. Joseph and Thirty Mile Point and left with no prefix.

I believe John is right. The amount of the “B” prefixes is around 400