Help identifying some WW2 exploded ordnance?

Hello all,
I was wondering if I could get some help identifying some exploded ordnance a friend found in Hong Kong. Does the roundish piece of metal seem familar to anyone?

At the site where it was found.

Close up.

Some background - these fragments was found halfway up Mount Nicholson which saw heavy fighting between Japanese and Commonwealth troops (mainly British and Canadian, but some Indians) who were entrenched at the summit of the Mountain. I found a .303 British bullet in perfect condition nearby lodged in the dirt, so I am fairly sure this was from the war. If I have my facts correct, the British did not have mortars or any ariel support, so if this is a bomb/mortar fragment it is most likely Japanese. Any comments would be greatly appreciated. Thanks!

A photo of the bottom of the threaded ordnance would be helpfull.