The internal difference is the block and how that all works together. The 602/603 both use the newer style block and is a much better design imo. The 600/601 block is still nice though, just a lot more dated by todays standards! The seals are also different between the old and new style block.
Other differences include the ratchet system on the lever, which as you say stops it flying open!
The stock also changed between the 601 and the 602, with the latter having a cleaner silhouette, and it pretty much hasn’t changed since then for the wooden versions! The mounting bolt position does change on the 602 but I’m not sure how far through the production run, all 603s seem to have side bolts front and rear of the action, which again hasn’t changed since!
The 603 is the only one with a stabiliser, or compensator, FWB were late to the game with this as Walther already had one inside the probe of the LGM2. The Stabiliser on the 603 is that odd looking black rod attached near the breech. To be honest I don’t notice the lack of a stabiliser on my 602
A minor detail but the later the gun the more modern the FWB branded diopter is that comes with it!