I am a retired Rover/Land Rover Cost engineer here in the UK and am well aware of all the vagaries of fixing on the old Land Rovers. The trouble is that the vehicle evolved in the 1940's using lots of parts which have their roots in the 1930's when BSF/BSW was the standard in the UK . When the Series 2 Land Rover was designed in the 1950's all the new fixings changed to what are referred to as Unified fixings ie UNF/UNC which basically copied the SAE standard. So most loose body fixing and the engine changed to unified threads , but the gearbox and axles stayed Whitworth . Gradually in the IIA era more parts, as they were modified, changed, to Unified threads . Towards the end of the S3 there was a move to go Metric as that was becoming the world standard . The 5 bearing crank 2.25 litre engine was one of the first , but you still had a mix of parts using Whitworth and Unified threads . The issue is exacerbated by budgets, or more accurately by lack of budgets! A new model programme using existing parts wont budget to change anything on them and and manufacturing wont have a budget either.
I came across exactly this at Ford when they owned LR . We were looking to use a Modified Ford Explorer front hub assembly on the LR3, which actually was made for Ford by Bosch in Clarkesville TN . We visited Dearborn and Clarkesville to see for ourselves and noted the wheel fixing were still inch sizes when the Ford World Standard was metric and Rover had gone metric 20 years previously. When we asked why this was the case the Ford design engineer explained that over the years they had tried to move to metric but the production lines made more than one model & the production engineers didn't want different but similar wheel nuts on different models due to the risk of a dangerous mix up. . The new model team would not pay for the work to change the existing model and the existing model team had no budget either , so inch fixing kept being retained, even then the rest of the hub was metric .
When you have an all new model, at an all new plant, you don't get these issues . That is one reason in the UK and probably the US, that Japanese companies, setting up on a green field site have an advantage over the established manufacturers.