You are so right!
The guy who ended up finishing the job is a real craftsman the main pin & though holding pin fit perfectly. When the measurements from the plan are cross checked they are all spot on. The main pin was put in upside down (I didn’t leave the old fitting as I wanted to scarf in some new wood – probably a mistake (in the end) not to) but because he had put the retaining pin in dead centre (main pin has a shoulder) it wasn’t a problem.
Forget fine art, sculptures & the like I could look at this for a long time – almost a shame it’s not going to be seen.