John,
You may have "nailed it." Here's the backside of the cover, with the registration cancels:

Same originating location. Roy Votaw was a philatelist specializing in getting autographs on his covers, especially of aviation pioneers. This cover has Hap Arnold's signature on the front. Does the resource you site just document the cancels or does it show the full covers? It would be interesting to know whether the cancel it documents was from a Votaw cover. I'm guessing that Votaw chose to post it from the Oakland location because of the "Airport Station" on the cancel, to tie it into the aviation theme he wanted for the cover.
I'd be interested in how he got Arnold's signature. (I have a display exhibit on the life and times of Arnold, hence my interest.) He settled in Valley of the Moon, CA, not that far from Mill Valley, after the war. But I think he was still in Washington at the time this cover was sent to his brother in Massachusetts. His brother was a Captain in the Army at the time, but not in an assignment where he likely would have had easy access to Arnold. So maybe he sent the cover back to Votaw, who got the signature after Arnold settled in Valley of the Moon.
IAC, thanks for the reference. It helps a lot to understanding the background to this cover
Basil
Added: I just took a look at sources for the book you reference. Nothing turns up at
Amazon, Abebooks, or BookFinder.Com. There is a copy of the 1952 edition on
ebay for $85 but that's a bit too much to risk not knowing if this cancel is documented in the earlier version. Might you do one of two things for me? If possible, just scan the section documenting the cancel, or if that's not possible then tell me what page it is on and I will see if I can get it from APRL. Here's the front cover, BTW:
TIA.