Brian, I'm not sure how I could tell. It dates from October 2004. As with all mark 1s, it has a brass bush at the breech, that is a sleeve fit over the breech end of the steel barrel, turned down to suit.
Mr Dink, I have done a load of tests on this rifle with various pellets. Sizing the pellets doesn't seem to do anything for consistency, compared to the 7.9s, especially the ones that were badged Mosquito Express. I always used them as a control pellet to see how the new variant or alternative performed. If I can find some scans I'll post them up.