The 97-2000 are often considered the "Goldilocks" for the Pro Sports. This particular rifle has the needed improvements to the cocking lever/linkage and the second stock screws, but retains the shorter stroke and smaller piston seal. The stroke and seal changes appear to have happened sometime after Serial # 1000, which is why these are sought out. I have no direct evidence of this other than this gun and the things I have pieced together from other owners and the internet. I love the histories of many of these guns. It's up to us to keep the information updated and share what we see. For example, in the past couple of years I have learned that there are almost no hard rules for the building of the amazing Theoben guns. Every time we think we know something, we find out we have more to learn.
From the AIRARMSANORAK page on the Pro Sport...
Over the years there have been a number of mostly minor changes to the Pro Sport but unlike the TX200, Air Arms have not designated these changes with a new “Mk”. However, some of the changes have led to reference to 2 “Mk’s”.
Mid 1997 saw a number of significant changes to the Pro Sport with what is commonly referred to as the Mk2. Due to some issues with the under-lever cocking system the linkage was altered and the piston stroke length was increased. A heavier return spring was used to close the lever more positively. The action was now secured to the stock with an extra screw either side of the stock, just in front of the trigger.
Among the other changes over the years were minor alterations to the stock design and a change to the size of the piston seal around 2000.
..Louie? 
