The W&S screw grip was as far as I am aware was particular to W&S . Other makers considered it it be bulky and unnecessary ,as did the likes of Greener who still promoted their cross bolt . Webley made a large variety of guns ,not all had screw grip nuts.
The barrel extension with or without a dolls [club,spade] head is a common feature of guns made all over the world ,each maker establishing a "house" style that they stuck to in some cases for generations .They also adapted and modified others ideas as their own . Scott's square "improved" cross bolt is IMHO a much better design than the original ,but guns are still built using round cross bolts .
The barrel extension in conjunction with what we loosely term a Scott style top lever if fitted properly will give a gun extra bite but this should not be confused with lever actions that use an extension with some form of bolting as with some American guns ,that I am not familiar with , we see very few in the UK .But the average made American gun has a longer action body than European guns ,this coupled with a top locking mechanism negates the need for the bottom bolt . Top bites on English guns were after the development of the bottom bolt always in addition and not the principle locking device. Hidden third bites and the eventual abandonment of top extensions in most guns was as much to do with cosmetic appearance as any thing else ,this is seen in later Webley guns that still had a grip nut but it served no useful purpose other than in the 50's through till there demise the cost of redesigning and re tooling was considered to great to warrant any further changes .