In May 1976, BMX Products began including a letter and a number with the C.
The additional letter indicated the month the frame was produced.
Scan the area to see if the letter C is etched above the five numbers.
The letter C stands for Chatsworth, the city in California where the Mongoose was made during the early years of production.
Im certain the dating is right from your first link so ive searched..
Thats the exact snippet of the tinterweb I was looking for.. Mongoose/Db/DK) - Son's Skatepark Bike Right, had a look, the ser number is HFFH03295 Which I think makes it..