The first .303 blank was the Rolled case Mark I, constructed on the Boxer principle and adopted in 1890. This was presumably done as an economy measure. The main problem of course was that it had to be chambered by hand as it was far too short to be fed from the magazine.
The long .303 blank appears to be an attempt to overcome this problem and allow magazine feeding. It is also a Boxer type case with an identical base to the issued blank. The body is heavy rolled cardboard into which is inserted a rolled brass mock body and bullet. The cardboard effectively forms the shoulder of the bottleneck of the case.
I have had this round many years but have so far failed to find any mention of it in the various records.