Hi Matty
It's vary hard to say if it's an SL800 from your photos, the stock appears to be an SL stock but most of JB's work was on the piston.
If it has an O ringed piston, piston weight and adjuster on the trigger then chances are it's an SL, the one thing throwing me on the pictures is the lack of screw on scope rail.
All the best Mick