Here's a 56 Plymouth 277 poly. Among other differences, the valve covers are scalloped differently than the Dodge poly. The later 318 poly is just a bored/stroked version of the 277.

(Note: some 56 Plymouths still had Dodge poly engines, as Plymouth was not geared up for full production of its new V-8. In 57, all Plymouths used Plymouth poly's)