Belgian Stamps With 'B'

Good Morning Everyone,

Like many on here, I have recently inherited a stamp collection from my grandfather. Unfortunately, it's currently thousands and thousands of stamps that have been stored in the form of bags and shoeboxes with no discernable system, so I'm doing my best to slowly work through by classifying the stamps by country and date of issue to get them into stockbooks. Prior to two weeks ago I knew absolutely nothing about stamps and I have to admit, it's kinda growing on me. I know next to nothing, but I'm interested to learn. I'm currently working through Belgium.

I'm running across a lot of Belgian issues that have a 'B' on them (see picture below), and are distinct from what I think are the original issues of these stamps. I can't figure if this is interesting and requires that they are displayed differently, or if this is nothing interesting and I need to move on. Does anyone know anything about this?

I recognize that these particular stamps have no monetary value, but I want to honor the family and finish what my grandfather started.

Thanks so much!

"Official" issues.

Look in the "back of the book" section.

Further clarification re the term "Back of Book":

Refers to listings such semi-postal, airmail, postage due, official, etc.

And for Belgium, this is an extensive section.

And "back of the book" section is called that way, because that is where they are located for single country catalogues, such as Scott US Specialized. However, in multi country catalogues, such as Scott set of worldwide stamps, they are located at the end of each country.

Most catalogues aside from Scott don't separate charity stamps. Many don't separate air-mail stamps. Some of Gibbons's catalogues integrate officials and dues with other issues, so that you see all stamps in their chronological order of issue.

Thank you so much for all the really helpful information, everyone!