No worries
As I'm sure you know, but for the benefit of others who may read this, once you've got the gun apart, replacing the valve o-rings isn't a hard job - three o-rings - one requires dissassembly of the valve. However try not to completely remove the tiny roll pin which holds the valve assembly together (leave it located in one side of the valve head). I did knock it out completely and it looks like it's going to be a bu**er to replace
I've never seen one, although Mr Clevely of Checkmate Guns (FAS guru by all accounts and very helpful on the phone), did mention that while he thought the 604 a lovely gun, he considered the 606 to be a "disaster" ....
cheers
Matt