Cancel Help - Capital City Mobile Po, St. Paul, MN USA

Any cover from Minnesota has a tendency to get picked up by me. This one was obviously mailed from Saint Paul, MN as the return address and zip code on the cancel corroborate.

Is anyone familiar with this "Mobile PO" cancel?

Thanks in advance for any assistance.

I would usually point someone to a postal truck or train but this almost appears as a doane cancel? Perhaps a private cancellation?
Here is a website (oddly enough from St. Paul, Minn.) That may assist? http://www.doanecancel.com/

Edit: A little follow-up reveals that neither a Capitol City and/or a mobile PO is listed in Minn. Although these aren't my area of study I could be missing something very easily so probably best if you have a look at the link provided to be sure. -Jeff

I Love Stamps: pointing some one to the Doane cancels is futile. The Doanes were used, for the most part, only by a 3rd and 4th class post offices in basically the first decade of the last century, over 100 years ago. It looks nothing like a Doane cancel: Doanes didn't have double circles; Doanes had number in the cancel bars, Doanes didn't have Zip Code identification, and Doanes had a completely different font type.