It's a Grand Old Flag Cancel
The other 3 known types of "seapost" flags, though, are of a different type, with the ship name in the flag instead of the field of stars on this example.
So, here's the story: The transfer of the "Seapost No. 2" machine from the San Lorenzo to the Borinquen took place in early 1931. Apparently the new flag die identifying the Borinquen was not ready when the machine was transferred, leading to the temporary use of a regular flag die. More specifically, Frederick Langford noted in a letter to me that the latest example of the San Lorenzo flag is Jan. 29, the first day of PO operations on the Borinquen was Feb. 28, and the Borinquen-named flag is first seen on April 30, leaving a maximum period of use of 2 months for this variety. And, since the postcard was mailed on the Puerto Rico-New York northbound trip, the changeover to the named flag could have taken place as early as March 23. This flag is unlisted in the Flag Cancel Encyclopedia, and is so far as I know still unique. Its discovery was reported in the July 1997 issue of the Machine Cancel Forum, but no more examples have been recorded to date.
Now, here's the best part--I found this in a junk box of postcards at a small local stamp show. The cost? 30 cents! As a unique flag marking, it could be worth up to $300 or more.
If you enjoy flag cancels, I strongly encourage you to look into them as a collecting specialty. While there are some very rare items (as indicated by the one above), there are thousands of flags that you can acquire with patience at less than a dollar apiece. You'll find plenty lurking in junk boxes at shows. For more information, please contact me.
While I'm at it, a little history lesson: The earliest use of a flag cancel was in Boston in late 1894. An example is below!