Weldsol, re: post #227. I understand from Federation of ME socs papers that after welding one side of the joint (obverse) the weld root needs grinding back as prep for the weld on t'other side (reverse).
For the final end plate, when you can't get to the reverse, the joint has a full penetration beyond the plate.
Any crack behind the weld will trap water and encourage corrosion, which in turn will develop stress concentrations and stress cracks so no "Un-welded gaps" are permitted. This is not the same as air receivers, so air receivers must not be used as boilers.
But I am no expert, just a "reader of rules"...
K2