[jae471]

The SS assigned "PIN" numbers to be used as transponder codes to pilots who were based at Hyde, Potomac, and College Park in February.

Here's the AOPA link: First step toward reopening Maryland airports

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[AmericanInTokyo]

What's also interesting is that not only American military aircraft actually got into the action. The first indication was the Germans getting aloft from Andrews on NATO 10 (E-3B AWACS Geilenkirchen GE). Allies at work.