As this shirt label states, it is made from Ban-Lon- a synthetic fiber
It was popular in the 60's and 70's and advertised as a material that you could wash and dry and yet the material would still remain in good shape without shrinking.  They claimed it would resist moths and mildew.  Ban Lon shirts were
made popular by Kramer on the TV series "Seinfeld" so you know they are very comfortable!

All original.  Sleeves and waist have a band around each for a slightly snug fit. Button slide welt pocket chest. Polished buttons. Size tag is medium. More to today's Small muscle fit. Light 4mm circular surface stain low by hem. Difficult to see-  see pic. See below for measurements

19" Pit-toPit

24" Collar to hem