Peter,
Found a reference to extended buttstocks in the "Collectors Only" book SMLE (No.1) Rifles Mk1 and Mk lll, Volume 1, 3rd Edition , Revised. by Charles R. Straton (North Cape Publications, Inc).
On page 74 of this publication Mr Straton observes that some buttstocks (specially on Britishicon made rifles in Australianicon service) had 1-½ to 2 inches of wood carefully dovetailed and fitted to the rear of the buttstock. He speculates that this might have been a "field" response to the need for longer buttstocks. He also notes that these modified buttstocks are the same length as the "Long" which were in short supply.

The buttstock on my rifle is the same length as a "Normal" so I guess there was a scarcity of the "Normal" length too….

I have attached some additional pictures of markings on my rifle and some more of the extended buttstock, with and without the buttplate.

D.