More like a 7.62 x 39 necked down and slightly blown out to a slightly longer 6.5 cal w a SR primer.
Yes, it is the 7.62x39 necked down and "improved" with a small primer. The case is not longer.
The Grendel came from the PPC cases, which came from the 220 Russian, which came from the 7.62x39. One can easily make Grendel brass from 7.62x39.