I've know of no other companies to use the archaic numbering system of old as Whamo used to do. A 40 mold is all Whamo and nothing but Whamo. Night Flyer not a Midnight Flyer as was available a while later. This baby was purchased at the Wagon Wheel pay to play disc golf course back in 1978 if memory serves me right.
The numbers had nothing to do with its weight. The "Night Flyer" was discontinued before the Spring of 1979, when I first got the DGA discs, which were then called Midnight Flyers. Considering that a stock 40 mold weighed in at a hefty 119 grams, a 126g disc would not have given the thrower.
1978 sounds like the right date to me. Where is (was) the Wagon Wheel, and what course was it close to?