Some makers only used the 3rd fastener on heavy load guns, others barely ever build a gun of any quality without.
I think it is simplistic to correlate a 3rd fastener with a level of quality: as Dan S.W. says above, it is about what was accepted procedure in the area that the Maker inhabited, in time, geographical and industry areas.
Purdey only used the hidden 3rd fastener, which they patented, but they are not restricted to only heavy load guns: many customers simply requested them in their game guns. It was an option, nothing more nor less.
Blanch used a top extension extensively, in fact the absence is the exception rather than the norm but many of those guns are undoubtedly 'Best'. They also had through lumps on many of their 'Best' guns but that is a whole other wasp's nest!
I don't think I have ever seen a Greener without a top extension but I wouldn't advise telling a Greener nut that their 'Best' gun isn't!
I don't think it is a plus or minus feature in general, it depends on the maker and your personal prejudice!
