The incomplete RBBB is not an actual "block," but a delay (we're talking about milliseconds) on the conduction of the electrical impulse through the right branch of the His bundle ... well, you know:-)
The modern, digital ECG machines indicate "abnormal" just because they have been setup with a range of data for the duration of the various ECG waves, generated by the electrical activity in the cells of the cardiac muscle.
Years ago, in cases of IRBBB doctors would recommend a more thorough cardiac examination to rule out morphological or functional defects of the heart muscle. Over the past few years, the need for further investigation has been waved in view of improved understanding of electrocardiography.
Like many other pilots, I have had it for 59 years ... always held a class one medical.