Just a few thoughts.
Dum Dum is nearby to Calcutta, West Bengal. There was a munitions factory there and it did make bullets for the British Army, most famously the dumdum bullet!
You say you have Guy's wedding cert, does it have Walter's occupation listed? How do you know he was in the Army?
What about his mother? Have you got her name ie. maiden name?
Were there no brothers or sisters? Where was Guy brought up?
If you have any more info post it and someone may be able to help.