Venom would finish their mods with a brass insert, not sure about shrouded barrels though.
Even Webley's figures for the Tracker wouldn't put this rifle at full power, so what you have is a well tuned, bull barrelled or should that be, shrouded barrel tracker with a custom stock, who made it is another question though ?
There was a Bull barrelled Tracker featured in a Airgun World mag back in the 90's, this wasn't the one though as I think that was .25 cal.