Tim, the images of the cap are extremely interesting, and have got me thinking ..... How sharp is the lower edge of the main body of the cap? I am wondering whether the cap originally stood proud of the nose of the projectile, with some form of spring mechanism that allowed the cap to be depressed, returning to the upper position once released. This would then function as a cigar cutter. In the upper position there would be three gaps, into which the end of a cigar could be placed. Then, with a sharp depression of the cap, the end of the cigar would be cut off, falling into the cavity below.
This would sit well with the match-holder/striker theory. Perhaps these were sold in pairs (match-holder + cigar cutter). I have a number of 1-pounder projectiles that have been made into smoking accessories. Cigar lighters, cigar cutters, match holders, inkwells, candlesticks .... these were some of the favourite modifications of bring-home souvenirs from the Anglo-Boer War. Just an idea.