Note to self: Remember never, ever, assume anything.
Just because the bolt removed from the top harness mounting point is 5/16ths, it doesn't mean the hole they came out of is actually 5/16ths
At some point, someone used a combination of washers, rubber spacers and nylocks to allow a Willians harnesses, with a 5/16ths hole in the top mounting point to be 'mounted' in a 7/16ths hole with a 5/16th bolt.
The top mounting point is 'T' shaped on this particular harness, making it very thin where it bolts to the car. You couldn't open the hole out to 7/16ths there would be so little meat left on each side. A quick play with a micrometer says it would be very approx 3.8mm (or 5/32") left on each side if drilled to take a 7/16ths bolt. Hence the rather unorthodox mounting technique I assume.
Relived to have got to the bottom of it, not sure I would have wanted to tested it in an accident...