Hi, I have had buildings 911, 913 and 914 since the 1960s. I am unsure about the scale of those kits. I have the box for 911 and I found pictures online for the 913 box, and apparently the scale is not stated anywhere. Does anybody know? Are HO buildings' models (at least from that era) prefixed with "B-"?
Thank you,

Some Faller models were deliberately made smaller for forced perspective, typically models of buildings that would be in villages high on the hills or alone in the alps. It was stated in the catalogue that this was the case.

I could never work out what the logic about prefixing some catalogue numbers with 'B-' was. There seemed to be no rhyme or reason to it.

No.
The B means that it is a kit ('Bausätze' in German language), Faller also had already assembled houses in their program. ('Fertigmodelle' in German language).

These 'Fertigmodelle' were assembled in the living rooms of many home workers in and around Gutenbach.
The high cost of wages for these home workers eventually influenced Faller's decision to start selling kits, instead of the already assembled houses.
The trick was that Faller let their customers to believe that it was more fun to construct the houses themselves.
And the trick worked, since then model hobbyists around the world are devoted to the glue.