I don't remember this label, but most of shirts I saw/wore in the '70s were either a discount store button ups or mail order ST, SW or LoTR tees. The design is perfect for Starsky, comfort with a little skin on display. Maybe his solution when upper brass kept telling him to button up his shirt? Or PMG's choice because the brand label made it an actor in-joke?
P.S. So when are you going to give us another of your steamy art pieces based on that basketball shot?