In Search of the Golden Skull, The ROM file of this game exists, but has not been released, most likely due to copyright issues.
Mattel asked Video Soft to do In Search of the Golden Skull instead of their normal Atari 2600 contractor APH. The game was never completed and only a playable demo was produced. The game was most probably cancelled because it would have required the use of a 16K ROM and 2K RAM cartridge, and was a 2600 product only. Mattel didn't want spend that kind of money for only one version of the game so it was quietly killed off.
Description: 'You're engaged in a worldwide search for a priceless artifact - a solid gold, prehistoric human skull. Use the treasure map to determine the Golden Skull's location, then begin your perilous search. Along the way you'll encounter air battles, steaming swamps and the pyramid maze. How far will your observation take you?'