Hello Thomas.
 
I have the same book(s) about Buckfast Abbey and I can offer the following:
 
The hives shown in the old photographs (early this century) have pitched
roofs, which were common then and a main feature of the traditional British
"WBC" hives.  Those are the ones with gabled outer coverings, which are
very attractive and look like little white houses.  They gradually fell out
of favor due to their complex and expensive design, and the difficulty in
moving them.  (They are actually a frame hive within an outer covering of
removable boxes.)
 
Now, the "modern" hives used at Buckfast are a modification of the Dadant
hive, which is deeper than our Langstroths.  That way they run all the
colonies on the single, large brood boxes.
 
Br. Adam evidently decided to keep the pitched roof idea, and so the modern
hives there have a slight peak to them.  These are used with an inner cover
("crown board") or a "quilt" which is the traditional carryover from WBC
days.
Back then they would use a sheet of canvas or other durable fabric to lay
over the frames, under the lid.  That has widely been exchanged for a
proper inner cover of wood, like we use.  The inner covers can then used
with bee escapes to free the supers of bees at harvest time.
 
I am not sure whether there is a dead-air space or insulation in the modern
Buckfast covers.  I don't think the books are clear on its design, either.
To me it seems rather elaborate.  A plain telescoping cover suits the
requirement just as well, and since hives are usually tipped forward
slightly to encourage the draining away of moisture, rain or snow will
eventually run off even a flat cover.
 
I hope this is somewhat helpful.  The Buckfast covers do look nice, and as
I recall the design was further intended to allow supers to be set down on
the cover (placed on the ground).  The front and back boards supply an even
surface for supporting removed supers.  This is also done with simple flat
outer covers, by inverting them on the ground, so the supers can be set
across on the edges.
 
Maybe someone who has been to the Abbey and seen the hives being worked
could offer us some clear details.
 
Best regards,
JWG