Apparently on Mario 64, in the courtyard at the back of the Castle (where all the ghosts are) where the sign at the fountain is, It says 'L is Real 2401' or 'Eternal Star' depending the way you look at it.

This has put up rumors that Luigi is in Mario 64, because L would mean Luigi and they would say he's real.

People also say that this means That Luigi is real in Paper Mario because 2401 is the day (2/4/01) when Paper Mario was released in the US.

Some people also read it as Eternal Star, which would make sense seeing that it's below a Concrete Star.

The text underneath the statement is Illegible, therefore puts no more info on this egg.