Apparently by joining the existing TF (rather than the unequipped Kitchener Battlaions) he could have signed up for the overseas commitment which would entitle him to the badge:
https://www.researchingww1.co.uk/imperial-service-badge
This would explain why he has a late enlistment date but is wearing the tablet.
As for the badge I think it's likely to be old stock, (albeit incorrectly unvoided and perhaps why they were still in the stores in 1914), rather than a new order using old dies. If you put a new order in then why not use the existing die?
There was a cap badge shortage in late 1914/early 1915 because manufacturers could not keep up with demand. You see a number of 1915 photos with new recruits wearing obsolete badges being worn for this reason. There are a few on the forum including photos of pre 1913 black RIR badges, Norfolk pagri badge on peaked caps etc.