TS
The Wessex inherited that configuration ( as did the Gnome Whirlwind) from its piston-engined Sikorsky predecessor.
The S-58 had a big Wright radial , with associated drive, gearbox etc trains up to the rotor system.
Westlands replaced this with the Gazelle turboshaft and later the twin Gnome installation , perforce having to use the same location.
IIRC the Wessex nose doors came out of the same basic jig as those of the S-58.
The twin turboshaft over the cockpit ( "normal" ) configuration was first instigated into production by Mil with the Mi-6 "Hook" in 1957.