Penny stamped G93

[pennypincher 0]

I came across this 1916 penny recently.  It's stamped G93 on both sides and maybe has w ar department arrow on it too.  Can anybody advise on what this is about?

[1949threepence 3,368]

Many old pennies are similarly stamped with numbers, names, initials etc. Never entirely clear as to precisely why.   

[pennypincher 0]

Thanks, it's the first one I've come across so at least I know it's not unusual