Peak Signal Station with Cannon

A rather unique photo (3.5" x 3.5") of the Peak Signal Station (Flagstaff) with the peak cannon which was known to be used as the mail gun, and maybe also as the typhoon gun.  Thanks to Stephen Davies, the flag is recognized to be a Jardine flag.

This cannon is currently kept at the Peak Police Station (http://gwulo.com/node/4892).

Any idea of the possible date of this photo (1920s is only my first guesstimate), and if the same cannon was used as both the mail gun and typhoon gun at the same time?