HPM 4 series outlet of 1960s vintage with neon “power” indicator
Looks like this outlet would require an exceptionally narrow box, one about 3cm wide and about 3 inches tall. I don't think I've ever seen one that size.
This type of outlet is known as an "architrave" fitting. That is, it's intended for mounting in a door/window frame or skirting board without a box. Even the standard size fittings are often mounted without boxes. Generally, boxes are only used in with brick or concrete walls. Hollow walls like Gyprock or Masonite use either a thin steel bracket with tapped screw holes, nailed to a stud behind the wall, or for retrofits what's known as a "plaster bracket" which is a similar thing but hangs on to the edges of the rectangular hole with two clips. If it's a timber or plywood wall the fitting is usually just screwed in with no boxes or brackets. Retrofitting onto concrete or brick walls is a pain, so the easy thing to do there is use one of the many size mounting blocks.
