In my, admittedly limited experience, I don't think I have seen such a gentle curve which would indicate to me natural colours.
"As a result the transmission graph and colour rendering are looking uncommonly good for a roof instrument costing around 300 USD"
I thought I found that the US warranty was all-singing-all-dancing. And requiring no proof of purchase. If the latter applied in the UK, eBay purchase would be good, but that seems not to be the case: 30 years though, and that would see me out!