Thanks to Jim from Downingtown and the good people from Baseball Reference, I now know the pinch-hitter extraordinaire was acquired from the Indians in November 1972 for Roger Freed and Oscar Gamble. (Minor leaguer Terry Wedgewood also came to the Phils with Unser, but he never made it to the Majors.) He was the team's regular center fielder in 1973 and 1974. He appears as an Indian on his 1973 Topps card, but as a Phillie on his 1974 and 1975 Topps cards.
Following the 1974 season, Unser was part of the 6-player trade that netted Tug McGraw from the Mets. He came back to the Phillies as a free agent in March 1979, and he scored the game-winning run in the 11th inning of the 1980 NLCS against the Astros. A baseball lifer, Unser is still employed by the Phillies as an advance scout.
EDIT: I just realized Night Owl is up to card #134 in his 1975 Topps blog, and you'll be seeing this Del Unser card again very soon. I believe this is an example of the "printers ink" color scheme.