I came across one in a market town charity shop just before last Christmas.  A bit like a vintage Madras jacket in overall appearance, half sleeves, label that suggests shtick, but at four quid for a summer shirt seemed not unreasonable.  Looks okay with jeans and desert boots.  No information as far as I can tell - Ebay yields one other.  They seem inexpensive - which is fine by me.  Do any of the American posters know anything?