(‘74) 318/904 complete Engine and Transmission

Is there a connection with the “-15” and not being able to locate information on the 4006730 casting number anywhere?

Ron, I wouldn't think so. All the "15" means is that that was the 15th run for that given mold. Molds usually didn't last for that many runs. That's the highest number I've ever seen. That's not a bad thing though. It just means the mold lasted a long time.