I do not know how many models of caravan were produced by H.B.H., but I can attest the fact that, apart from the "Series-1" and "Emperor" caravans and those unnamed versions, there was at least one other model. When H.B.H. ceased trading I guess that the partners split the remaining unsold caravans between themselves as my father took two identical caravans, both of which were then located at Rose Green Caravan Camp, Bognor Regis. One became our holiday home and the other he eventually sold to a colleague in the PRV drawing office.
The caravans located at Bognor were certainly a derivative of the Series-1 as they were almost identical except that the double berth was at the towing end (the door therefore being forward of the wheel arch).
Since all three partners were steeped in commercial vehicle manufacture the caravans were constructed in a similar fashion to the coachwork found on buses of the period; except that the beech frames were covered in aluminium sheeting rather than steel. Steel construction would have necessitated a more powerful, and more expensive, towing vehicle
Aluminium wasn't used on buses until the mid 1950's; the first London Transport bus to use aluminium was the Routemaster and just one of its advantages was in its running efficiency due to its relatively lightweight construction.