It was a special model for taller riders, hence the name "Longfellow". In 24" and 25½" sizes only. Yes, it would be 531 PG (plain gauge) for heavier/bigger riders. When Raleigh took over the Carlton name in the early 1960's, the serial numbering system got pig-eared, hence not easy to date models up until 1973. There's some pictured on Instagram and occasionally pop up on Facebooks various vintage bike groups.

The one posted above by @bertinjim is rather nice example of this rare model, with bullet silver finish, yellow & sky blue bands with chromed fork socks and fork bridge.