I have seen a few 6004’s through pony club (low use, carefully used) with loose pins, oval pin holes that just suggest that the frame metal is just not as good, the triggers are also just not as good. So these 5-6 year old guns are already showing signs of wear whereas 40year old 604’s generally just need a £6 set of seals if not working and maybe a coat of paint if badly worn. The old 604’s definitely had more hand finishing on the internals. Where there are breakages on the 604’s it’s usually the top cover where it sweeps up over the grip and that is usually as a result of being dropped.