In addition to Andersen Air Force Base and Naval Base Guam, there was also a NASA Apollo tracking station there at the time.
And as the OP says, not exactly a ton of tourists. So while military provenance can't be ascertained, it's not a bad bet.
I have a Straight Writing delivered to Guam, very close in production to the OP's as well.
Cheers,
-GW
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