This block could be considered a mystery block. Chevrolet documentation shows the block being used as early as the 1964 model year in 300 plus application and commercial trucks. Verifiable proof of this model usage was not found until 1966. This block was used interchangeably with the #3858180 block in some applications. Passenger 327-275HP 2-bolt, Chevelle 327-275 &350HP 2-bolt in March 1966 introduction, and Chevelle 327-300 & 327-350 2-bolts.