While I have post some info about this topic in Horn Articles Online, one topic I have not yet posted on in this site specifically is the B-flat marching French horn. There is certainly confusion out there as to how this instrument might be used in groups, how it is notated, etc., as I frequently see it mentioned in the search terms in the site statistics.
Good news for those searching for information! The episode for this week of The Mellocast (episode 57) is on the topic of the B-flat marching horn; the main feed is here.
I also discuss the instrument briefly my new book A Mello Catechism. In short it is an alternate to the mellophone for marching groups that takes a horn mouthpiece and uses the fingerings of the B-flat side of a double horn (exactly the same fingerings as “thumb down” on a double horn!). Check the links for more information.