I think you will find that PLANET was the name of the firm that made the loco. It looks to me like the makers name from the front of the bonnet of a small diesel loco, the sort of thing that would have been used in and around factories, water works, quarries, etc.
Just googled Planet Diesel Locomotives and it came up with this:-
http://en.wikipedia.org/wiki/F._C._Hibberd_%26_Co.
And another link, scroll down to the picture, Fig 7 and you will see where the plate fits on the loco:-
http://www.irsociety.co.uk/Archives/14/Planet.htm
Regards, 62440.