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.