This post is in answer to a comment I received from my last post from Hong Kong. I must admit I was pleased to see my blog entries being read from so far away! Anyway unfortunately I don't think I can really answer Kwong's questions in much detail but the following is as much as I know on inter-war design for pillboxes. The 1936 RE Manual gives three drawings for concrete machine gun emplacements, the first being the familiar 1936 Design referred to in Kwong's comment, variations of which were used in pillbox construction in the UK during 1940 (esp in Norfolk, Taunton Stop Line, GHQ Line in Surrey / Kent) and as Kwong suggests in Hong Kong as well.
Above: The familiar 1936 design for a machine gun emplacement.
The other two examples of concrete machine gun posts include one of which is built into the shell of a house. The other is described as an emplacement with medium cover where concealment is necessary. I don't think this was a practical design (although please feel free to correct me) because as far as I understand it, the air space in this design, which was intended to help in the absorption of the shock of shell impact, was very hard to achieve in construction.
Above: Top - concrete machine gun emplacement design for construction within the shell of a house. Bottom - concrete machine gun emplacement where concealment was important. Note the air space, designed to reduce concussion from shell blast.
The manual gives several other designs for machine gun emplacements but these were all field work emplacements designed for trench systems.
Above: Three designs for machine gun emplacements which would have been incorporated into trench systems.
The 1933 Manual Kwong refers to was a Field Engineering manual - i.e. only deals with field works and not concrete emplacements, which were only considered necessary for protracted defence.
The 1925 Manual Field Works does refer to reinforced concrete emplacements dealt with in Military Engineering Vol II (Defences) but unfortunately I don't have a copy of this.