I have seen this idea illustrated and described in German beekeeping
manuals. It is a narrow chamber across the front of a hive body and it
holds one frame. Imagine you're looking at a standard hive body, head on.
The idea would be like a double wall on the end of the hive body facing
you. There is a space between the two walls into which you can slide a
frame. This frame would be positioned "sideways" to the regular set of
combs in the hive body, and separated from them by the "inner" wall. I
suppose this inner wall has some set of holes or something so the bees have
easy access to the "building frame." The outer wall facing you would have
a pane of glass with a removable cover. Then to estimate the progress of
the colony you just take off the glass-covering panel and you can look in
to see what the bees are up to on the face of the comb. This gives you a
relative idea of what's going on in the main set(s) of combs. It makes for
a longer hive front - to - back. I have never seen the idea developed in
US/UK books. I hope this is helpful.