I THINK it's a Bianchi 500, but I'm not sure.  The ad certainly makes it clear with the big pushrod tube on the side along with the weird, finned portion of the case that protrudes forward towards the front tire.  However, there are a couple of key details missing from the ad that I see in the picture.  The photograph has a jockey shift with a notched guide on the rider's right side of the tank. It also doesn't show the pushrod tube or the chrome rocker cover on the head. The front end isn't quite spot on, either.  It's hard to tell the difference between what's artistic license and what is lost detail on an old photo.

In my mind, the search continues.