I started to plan a comparative analysis to find as many variations in the 1787 shilling as I could find and work out their combinations to see if I could notice a pattern. Before I got more than toe deep I decided to look into the meaning of the reverse legend. Stumbled across this amazing article.
It explains everything! Answers all my questions. From the conclusion:
"These coins represent an intermediate stage in the modernization of coinage techniques. The inscriptions on the obverse dies were made with individual letter punches, a centuries-old practice, but the reverse dies were sunk from fully-lettered punches, a major advance. Even on the reverse dies, however, fine features such as stops, the Westphalian horse, the semee of hearts in the Hanoverian arms, and the strings of the Irish harp were added individually, suggesting that truly complete punches (except for the last two date digits) remained difficult to manufacture or impractical to use".
This explains the differences I was seeing in the positioning of the inscriptions on the obverse. I also noticed that some examples had a six-stringed harp and others had a seven-stringed harp on the reverse. I can imagine some poor bugger trying to hand-engrave evenly-spaced strings on various die. Not an easy task I would imagine.
Boy this hobby goes deep! I could spend a lifetime trying to track down strikings from all the different dies of this coin and a mint finding a Pingo proof. That in itself would be another rabbit hole, collecting a complete set of these 1787 proofs of shilling, sixpence, guinea and half-guinea! I can see much fun awaits me!
Please correct me if I misuse jargon.