The Great Game Machine, whilst being a sought after “Oldie” is not itself rare. They appear reasonably often on USA Ebay.
The Steinitz module, being the strongest module for the GGM and Modular Game System, is scarce. I have seen perhaps five on Ebay over as many years. The carry case is also a rare addition. Together with the original boxes the package just about justifies it’s inclusion in this Rare feature.
Thanks to Dan and Kathe Spracklen’s strong program the original Modular Game System and Sargon 2.5 module was an important advance in the development of chess computers when it appeared in late 1979. With the Steinitz module the Applied Concepts / Destiny company had improved on that machine by 100-150 Elo points and included many more features, including position evaluation (photo bottom right). However the improvement had taken over 3 years in a rapidly changing market and the buying public had become wary of new modules with extravagant Elo claims. The GGM was also showing its age compared with the new machines like the Fidelity Elite A/S. So the Steinitz did not sell well, hence its scarcity today.