The explanations below does not necessarily apply to all stamps, because the press sheet configurations/markings changed over time.
Quote:
The line on the RHS? any ideas? nomenclature?
I have only reference to "Centre Lines" as a guide
in dividing a sheet of stamps.
In this particular case/configuration, the guide line is also the center line; although that is not always true in other cases.
Quote:
Would this may be a booklet pane stamp perhaps?
No, it is still a sheet stamp. The original press sheet of 400 was cut into 4 panes of 100 stamps. The original sheet was cut roughly along the guidelines (centerlines). There is a straight edge (no selvage) on the side that is cut; depending on the cutting accuracy, you may or may not see the guideline. Therefore, the pic you showed is often called a straight edge copy. Normally, it is considered less desirable. It affects the value so much, that it is often the target of reperforation.