Can't find 850 mm wide radiator. Not sure why this size of radiator was used in the first place. Checked other radiators in the house. They are of sizes 550mm, 750 mm, etc. Nobody seems to sell these.
Pipe locations I'd assume. Perhaps it's being measured including the fittings? Can you show a photo and indicate or describe where you're measuring to/from?
But it may be an imperial/metric difference. In which case a plumbing wiggle may be needed, possibly involving floor access and moving one or both pipes.
Be warned... if this one is leaking then your others may be joining it soon. How old are they? Have a look at them all, if they're all going crusty on the seams then it may make more sense to replace the lot, get it over and done with and don't have to worry again.
While you're at it get a filter fitted, and ensure decent corrosion inhibitor is used.
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