I’m in touch with John Fritsche, who posted an Edhat article about the SB Airway Beacon towers here back in the early days of Air Mail. He’s seeking more info and details from anyone familiar with these topics, and today sent me this photo of a letter sent on the inaugural flight.
As you can see (click the photo to see a larger version), there’s a stamp at the left showing the airmail route. Here’s a closeup with the contrast enhanced.
If anyone is interested in these topics or has more info, please post it in a Comment here. I’ve also invited Mr. Fritsche to join this discussion. As you might guess, he now has quite a bit more information than was in the Edhat item and the comments there. I’ve suggested that he write up a summary and post it here so that we can gather and share more details about this interesting time in SB aviation history.