From 1955 through 1957 on trucks (their V-8’s all were 265 cubic inch during the first years) Chevrolet added engine cooling differently.
The radiator was moved further to the rear due to the V-8 shorter length over the 235 inline six cylinder. To force more air into the radiator core Chevrolet added an upper and lower metal spacer plate. These were attached to the grill and radiator support to fill in the created space. The enclosed drawing is from a 1955-1959 Chevrolet Truck Assembly Manual.
In recent years these plates have become very difficult to locate. Over many years, replacement V-8 engines (1958 and newer) were given the more traditional round shroud to help radiator cooling. Unfortunately, once these two parts become separated from the trunk, new-comers that were not there when the plates were removed, have no clue of their purpose.
More landfill material!