I’m not sure if you made any progress with identifying the relevance of the number, but many support units had four digits. I did a quick search and found the 3217th Signal Service Battalion, which was stationed in England for a time.
The three bars certainly remind me of Preparation for Overseas Movement (P.O.M.) markings which were used to mark individual/unit equipment and vehicles to identify it during shipping. A groundbreaking new book was just published on this subject called “Destination: Victory”
www.pomcodes.com and some cursory information can be found at
www.questmasters.us/POM.html
-Paul