When/How/Why did Jurupa Peak get flattened?

The high point of Jurupa Hills has a flat surface on the peak as a result of some manual flattening, probably many years ago. Does anyone know the true when/how/why of the reason for the flattening? I've heard speculation that it was prepped for military or radar installation that never subsequently happened or that it was somehow connected with the Ontario Airport. While those speculations sound plausible they give no clues about when the flattening happened. Does anyone know any details? It could have been flattened in the 1940's for all I know or maybe the 1980's.

2N17X Jeep Trail is now closed

Because of the Pilot fire of August 2016, the 2N17X trail east of Silverwood Lake, near 2N33 (Pilot Rock Truck trail) is closed and will likely be closed for at least a year, possibly as long as 5 years. Because the Pilot Rock Truck Trail road (2N33) is so busy, it will probably be opened within a week or two after the fire is out.

Very sad because this was a great trail. Moderately difficult and only takes 2-3 hours to complete.