I stopped by one of my local gear sources, and found the proprietor had come across a batch of these:
This is a US GI Issue Wool Field Shirt. They were issued from the 1950s up until the 1980s. Got my first one from a sympathetic supply sergeant at my first unit. He managed to get a bunch in assorted sizes just before they were declared obsolete, and gave them out to those who knew and asked about them (and didn't piss him off).
Despite not being standard issue for almost 30 years, they remain available via Milsurp channels, although sizes larger than Medium are a little less available. They are a well-made, warm garment that will last for decades.