The keystone is the stone pillars at Gobekli Tepe in modern day Turkish. This is the emblem at the top center of the Bathhouse in Aleppo. This is the stone which honors the bull and the alligator by being painted on the side of the temple at LSJ.

I did a search through ancient architecture. In most ancient structures huge beams rested on pillars, like at Stonehenge. But Goebekli Tepe is different. You see stone pillars around the perimeter and a single support stone in the center of a circle. The keystone. Arches of smaller stones were held in-between the circumferential pillars and the center keystone were built from smaller pieces of hewn stone, since fallen to the earth and lost or used for other projects. It's not obvious, because it about seeing what's missing from the picture. The missing stones were formed into arches when the structure was originally built. This was a gift from somewhere/someone else. The stones appear to be marked with the sign of the giver, in their honor. I see a bull and an alligator. I think we know who provided that secret knowedge, and how this whole story began.

Göbekli Tepe (Turkish: [ɟœbecˈli teˈpe],[1] Turkish for "Potbelly Hill")[2] is an archaeological site in the Southeastern Anatolia Region of Turkey approximately 12 km (7 mi) northeast of the city of Şanlıurfa. The tell has a height of 15 m (49 ft) and is about 300 m (980 ft) in diameter.[3] It is approximately 760 m (2,490 ft) above sea level.

The tell includes two phases of use, believed to be of a social or ritual nature by site discoverer and excavator Klaus Schmidt, dating back to the 10th–8th millennium BCE.[4] During the first phase, belonging to the Pre-Pottery Neolithic A (PPNA), circles of massive T-shaped stone pillars were erected – the world's oldest known megaliths.[5]

More than 200 pillars in about 20 circles are currently known through geophysical surveys. Each pillar has a height of up to 6 m (20 ft) and weighs up to 10 tons. They are fitted into sockets that were hewn out of the bedrock.[6] In the second phase, belonging to the Pre-Pottery Neolithic B (PPNB), the erected pillars are smaller and stood in rectangular rooms with floors of polished lime. The site was abandoned after the Pre-Pottery Neolithic B (PPNB). Younger structures date to classical times.

https://en.wikipedia.org/wiki/G%C3%B6bekli_Tepe