As far as I can work out only one SL600 was released, Matty.
That gun was given away as a prize in AGW or AG, and there's a thread on here about it --- search for SL800.
Edit :- found it --- http://www.airgunbbs.com/showthread....69#post5694269
Obviously, JB could have built and sold others which weren't so well documented so perhaps Greenway could fill us in better ?