Cool 1970s men’s button up short sleeve shirt. 100% nylon Enro shirtbuilders fashion knits. The tag is marked irregular. I’m not sure why. Some small stains and tears/snags noted in photos. Stains may come out with proper cleaning. Rough measurements

Pit to pit 19”

Shoulders 16”

Top to bottom 29”

The nylon is stretchy and measured while taut.