13¢ Prexie Solo From Guantanamo Bay

Many thanks on my previous questions on usages. This one, I think, is straightforward, but some confirmation would be appreciated.

This is a 13¢ stamp sent from the US naval base at Guantanamo Bay on Sept 21, 1939 (a received backstamp has the year).

Is this 8¢ for airmail from Cuba to the US plus the 5¢ domestic airmail rate?

An image of the backstamp would be useful to know where/when (i.e., how fast to somewhere). It is a bit unusual *not* to have some indication of what service was expected for the postage.

Just a backstamp from when the cover was received in Minneapolis.

So, six days. I wonder if the lack of any "Via Air Mail" indication, caused it to go by ground from Florida, even though air service was paid for (if your postage analysis is correct). The delivery time should have been half that if it went by air mail the whole distance, even in 1939, right?

Does it count as a genuine solo usage of that rate if it can't be proven the cover traveled by air the whole distance?

That time period was exactly when GITMO was being expanded, and a massive construction effort was taking place. That year also saw the transition from a Cuban PO handling US mail from GITMO to the Naval Station PO. maybe that impacted the duration of letter transport?

See PDF pages 19-21:

http://www.philat.com/biblio/P/PIPE0800.pdf

The Beecher/Wawrukiewicz domestic rate book specifically lists a to/from Gitmo airmail rate of 10 cents per half ounce at this time period. To be a "full value" solo use, a cover must exactly pay real rates, which is not obvious here.
My first impresion, somehow this smells like a collector-submitted cover seeking postmarks and not quite knowing the rate, but making a guess. Also no return address. Was it ever sealed? It does not appear to be now, but is the flap-gum disturbed?
But digging further, looking at the time facet: Taking 6 days gives no sense of any airmail service. The timespan is more consistent with 3 cent surface + 10 cents (unmarked) special delivery, which would also be consistent with a backstamp, which was being done on registered, special delivery and general delivery at this time and NOT on airmail. This gets you to a valid solo use, although the lack of a "special delivery" marking reduces its desirabiity for an obvious rate transparency. It will be interesting to see what other explanations can be built on actual rates/services, etc.

The gum flap looks like it was sealed lightly just in the center. The letter was opened without tearing any part of the envelope, including the gum flap.

First that is a nice domestic origin for the cover shown in the OP.

Overall I agree with everything John Becker wrote. But wish to add a small amount more. The Rustad book, The Prexies, mentions the only domestic solo use was as described by John Becker, three cents non-local domestic first class plus ten cents for special delivery. The one shown in The Prexies, from only the front, was sent within about thirty days of the one shown in the OP, except it had three differences on the front which the subject cover above lacked, .a hand written Special Delivery, a USPOD Special Delivery service handstamp and the handstamp auxiliary marking noting the "Fee claimed ...." for the Special Delivery Service. That actually confirms the letter was attempted as Special Delivery matter by the office of addressee.

As to foreign solo usage, there is but one combination rate for sending a letter to Japan with internal movement with in Japan.

This value has few ways to be used solo as compared to most other values. However domestic three + ten letters were a common use such that solo covers are not scarce nor pricey. Solo foreign, if they exist are a different matter.

As parcelpostguy wrote 13c uses to foreign destinations from this period are quite hard to find. I just bought this one used a few days before the 13c Prexie was issued. Postage is 10c for single rate supplementary mail in New York and 3c additional rate for air mail in Europe.