As originally used in the Service, the Disc, Marking, Butt was made of brass and was used to tie the rifle to a particular formation. Commonly, the Regiment and battalion would be identified, also the Company to which the rifle was issued, followed by a Rack Number. The system did not go any higher than Regiment for Infantry, although formations such as the Service Corps would have their rifles marked to the Corps, then to a particular part of that Corps.
This all became rather controversial about the middle of the Great war and, finally, in the interest of secrecy, they stopped marking the discs at that time.
When a rifle went for FTR (Factory Thorough Repair: a complete rebuild). the Butt Marking Disc would be removed and replaced with a blank, which then would be marked afresh by the Regiment receiving and using the rifle. Generally, used Discs would be re-faced and used again; when they were no longer thick enough, they went for scrap.
There were not a huge number of 1908 rifles built and, today, they are a great deal scarcer than they were 106 years ago. If you want to make a lot of faces on this forum smile, you will post some nice photos of this piece for us all to drool over.