I purchased it on eBay from a private seller who upgraded to an RNS510.
I thought it was legit when I saw the VW box and original VW manual.
Everything seems to operate as advertised - just button layout is different.
(The seller was a VW tech who installed it for me in < 10 minutes)
PS Still hanging out for intelligence as to how to display time of day on the RCD510
I have seen this discussion...
http://forums.vwvortex.com/zerothread?id=4811536
...which discusses (amongst other subjects) setting the clock on the RCD510 and shows pictures of the unit displaying the time.