These chassis/engine nos have appeared in some registers as SXF803, and in others as 603. It's now clear that they apply to 603, and that any link to 803 was almost certainly a mis-reading of the number. 803 probably exists/existed, but this is not proven, apart from a reference in 'The Green Machine' to it probably being a Land-Rover.
To add (or alter) information on this page, please contact me on sxf@teeafit.co.uk.