has anyone come across this before 'Rolex double model number'

[dennyboy]

has anyone come across this before and i so could you please shed some light on it for me.
you can see 6494 lined out and then 6422 engraved underneath.

when i got this i was told it was a rare precision non-date early 1957.
i was told they used up some of the older slim cases before moving on to the newer cases and these are very rare to find.
once again this is just what i was told when i got the watch, i really have no idea