The alloy used in the Land Rover was Birmabright which is 7% magnesium, sometimes 1% manganese and aluminum.[1]
Without the maganese this would be a series 5 aluminum according to the alloy grade comparison I found.[2]
1. http://en.wikipedia.org/wiki/Birmabright#cite_note-Camm.2C_B...
2. http://www.practicalmachinist.com/vb/general/aluminium-alloy...