This very interesting advertisement was posted on the Unofficial Martin Guitar Forum (UMGF) website.
Wittich's Musical Shop in Reading PA was a regular Martin customer from 1929 to 1962 and made frequent orders for a briad range of Martin instruments.
This advertisement was published in the May 8, 1944 edition of the Reading Eagle newspaper.
The advertisement mentions that Wittich's had just received the last order of Martin guitars they expected to receive for the duration 0f the war. It turns out Wittich's had ordered three D-18 and one D-28 on December 1, 1943. A batch of D-28 guitars was finished on April 5, 1944 and the single D-28 was probably shipped as soon as three D-18s were available.
Luckily for Wittich's they able to place an order on August 18, 1944 for two 00-17, two 000-21, three D-18 and two D-28 guitars, so they weren't out of stock for too long.
Original UMGF thread: https://umgf.com/viewtopic.php?f=3&t=198603&p=2397089#p2397089
Even more information can be found in this article published in the Reading Eagle newspaper.