The "Switch Combination Aust Mark 1", commonly known as a Murray switch, are Australian WW2 issue made by Murray C S of Melbourne Australia.
There are 2 types of Murray Switches, one has a full Die cast body and the other has a steel body with a brass fastener to hold the .303 case in.

Photos and info © Scott Lynch.

The Murray switch used a special .303 case with a T cut in the neck of the case so that it can crimp down on Demolition cord.