According to my copy of British Enfield Rifles, Vol. 4, the Pattern 1914 and U.S. Model 1917 Enfield Rifles (Vol 4) by Charles Stratton, the compartment in the buttstock is indeed for a bottle of oil and a pull-through for cleaning purposes. I realize that a P14 is not the same as a SMLE, but as they are both British service rifles of a similar vintage...

Also, I tried doing a little research on the unit disk on your rifle, but didn't have any luck. I did find this reference, however:

The Identification of Buttstock Marking Discs in British Service by Peter Laidler

It seems to only cover unit markings for rifles in British service proper, not Commonwealth service.