Revenue stamps for postage !?!?

A cover from a military regiment that has been despatched from an APO . 

But what I found interesting was that a revenue stamp for ₹5/- has been used instead of a postage stamp ! I wonder if it’s the norm for military post !?!?!

Wish someone could clear this doubt for me.  

Update : 

I found from a retired Colonel that this mail is an OIGS mail - On Indian Government Service Mail . The stamp is a service postage stamp - exclusively to be used in communications from Govt offices !