1975-1978 Titleist Acushnet DT
The USGA began listing conforming golf balls starting in 1972.
#146 Titleist Acushnet DT, #2 Titleist Acushnet-Pro Traj, #1 Titleist Acushnet

These three golf balls all have 324 circular dimples arranged in a modified icosahedron pattern (four dimples were removed in each polar region for the name and number). Patents were filed in 1972 in both Great Britain(1) and the United States(2) for this technology.

These golf balls were three piece balls made with a small polymer or liquid center wrapped with a long elastic rubber thread and a thin resilient cover. Balls #1 and #2 had covers made from a soft material called balata. They show cuts and cracks on the surface. Golf Ball #146, which Acushnet first introduced(3) in March 1973, had a new durable cover made from sodium and zinc Surlyn. Spalding claimed this golf ball infringed their patent(4). The infringement trial lasted from 1-1-89 to 2-13-89 and the court ruled that Spalding had proven their claims of infringement. Acushnet stopped making this DT Titleist in 1977.

Found this from a collector online and his information was taken from the USGAs list of conforming golf balls.

Could be for a US team play or ball made in US. Hard to determine.