I have access to the original factory records, and in those the Webley Stingray is listed in the monthly production compilations, from May 2000 to October 2005, in the following ways:

Stingray
Stingray Std
Stingray Carbine
Stingray Carbine Walnut
Stingray De Luxe Carbine
Stingray K De luxe
Stingray K
Stingray Ex (Export)
Stingray D/L
Stingray XS
Stingray S/Bore
Stingray S/Bore with BBMF Silencer
Stingray Express (Export)
Stingray D/L Beech
Stingray W K
Stingray K De Luxe Beech
Stingray Low Power
Stingray K Low Power
Stingray K 10J (Export)
Stingray XK

The last serial numbered Stingray was 932998 on 22/10/2005. Obviously many of the above descriptions relate to the same spec of rifle.

The 'Stingray XK Kit' was introduced towards the end of production, at a price of £218.90, described thus:

Longbow type silencer fitted to a .22 precision carbine barrel.
Precision open sights
Beech fluted stock
Fitted with sling swivels
2 stage GOLD plated trigger
Optional telescopic sight and mounts can be fitted.

Still for sale well into 2006.

I can't find any mention of an 'Extreme', so would go with the 'shop special' explanation - ie Sandwell Sports offered custom versions? Would be interested to see that handbook, and the serial number - with that I can date the week of production?