We were as baffled as you were, sir. Which is why we enlisted the help of an expert. Christophe Loiron is the owner of famed vintage emporium Mister Freedom and the designer of the shop's in-house line of military-inspired clothing. He, of course, had the answer for us.
Most early American sweatshirts, from the 1930s and '40s, had a 'double V' neck construction on both the front and the back of the neck. That small V patch was made from a double layer of webbing material—ribbed cotton jersey also used for waistbands and cuffs—not the fleece or French terry from which the shirt's body was made. The purpose was to act like a sponge, as those shirts were used as athletic gear. Not very glamorous, I know, but just look at sweat patterns on shirts after physical efforts.
The reinforcement also controlled the stretch of the neck when the garment was pulled over the head. On later sweatshirts (the late 1950s, for example) the 'V' is still there, but it's usually a single layer of ribbed cotton stitched to the fleece. By the '60s, it often became just a flat overlock stitch on the collar, just for decoration. And before the recent 'vintage craze,' modern sweatshirts didn't have anything on the collar at all."
Mister Freedom's well-edited vintage assortment features Army/Navy goods and workwear from the 1890s through the 1970s.