You will find the reference to the neoprene seal in the 1956 passenger car shop manual supplement. That seal was introduced for the passenger car line, along with the 848 head. Late 55 block? You have to be careful There are late 55 blocks for the end of the 55 model year and there are 56 blocks cast late in 55 for the 56 model year. Those early 56 blocks used neoprene seals. Truck blocks may be a different animal. Pontiac blocks used the neoprene seal.
