I know that the 737 NG borrows the basic wing design of the 777, but I have also heard that BAC 4xx airfoil on the 737 classic (300/400/500) was an early supercritical aerofoil design?
Is this true?
Supercritical yes, but early? Not really.
BACXXX airfoil plot, shamelessly copied from airfoiltools.com.
The plot of the BACXXX above shows typical contour characteristics of supercritical airfoils:
however, this airfoil does not take those techniques to their extreme. It was a redesign on the original 737 wing, extended by 4.4% of chord at the leading edge and overall a compromise between high commonality with the earlier design and improved aerodynamics.
At the time when the "classic" series of the 737 was designed in the late Seventies/early Eighties, supercritical airfoils were already firmly established. They were first researched by K. A. Kawalki in Germany in 1940, who arrived at identical shapes as R. T. Whitcomb at NASA a quarter of a century later. The first operational supercritical airfoils, at the time called rooftop due to the roof-like shape of the upper surface pressure distribution, were employed by Vickers in the VC-10. This remarkable aircraft is to this day the fastest airliner with a maximum cruise Mach number of 0.89 and flew first in 1962.
Boeing became aware of supercritical airfoils only after the original 737 was designed, so the redesigned wing of the classic series carries some of the baggage carried over from the earlier design.