You can google German Pillbox World War 1 Hindenburg Line, and dozens of pictures will come up.
Thank you for pulling together the list of hyperlinks. In the first link, the following photograph is likely to be a dedicated MG pillbox from within which an MG team would operate:

It illustrates the principle of defending with an 'empty' battlefield, i.e. the position is not visible until you are quite close to it.
The next photograph illustrates the same principle, though in a different context:

It was very common for concrete hardened positions to be built inside existing structures. The nature of the aperture and it being close to the ground means that it is most likely an MG position and not an observation or command post.
This photograph is not an MG pillbox:

It is a command bunker with the open 'window' being a location for a visual signalling device that was used to communicate messages further back in the defensive sector.

is a Moir pillbox, which is a British design.
The images from the Australian War Museum are a mixture of command, observation, and hardened shelters for infantry (MEBUs), which will have housed MG teams but not been used as the positions from which the MGs were fired from inside.
Robert