The 604 originated in the 1970s which ceased production a few years ago. The 6004 is based on the same design but manufactured by a different company Chiappa Firearms on licence. It's basically the same gun but is mass produced with more pressed parts and less hand finishing than the original 604. The cost today is less than the FAS in its day.

A basic summary can be read here:

https://www.all4shooters.com/en/Shoo...essed-air-gun/

I also found this forum thread which is interesting and has comparison photos of the Mk1, Mk2 and 6004.

http://ttorg.targettalk.net/viewtopi...46533&p=221288

The Mark 2 604 appears to be a good choice as advised on here, as it still has the hand built finish but with a number of improvements over the Mk 1. Having said that if i don't find a good Mark 2, I may take a chance on the 6004 as it will be a new gun with a guarantee. If there are any quality control issues i can always return it for an exchange or refund.