The Oversize Vacumatic was introduced in 1934. It is one of the largest pens ever made by Parker, approximately 15 mm wide and 135 mm long - and certainly one that holds the most ink.
The first Oversize pens were opaque, but were replaced by transparent versions in August 1934. The Black Transparent version had longitudinal "windows" showing the ink level.
Click on the pen that looks like yours.
Early Oversize Lockdowns had 2-tone nibs engraved "USA PARKER".
The Black Windowed version was replaced by laminated transparent versions. Green was introduced this year.
Brown was added to the line. Nib engraving changed to "PARKER USA".
In 1937 the Oversize Lockdown was discontinued and replaced by the Senior Maxima , which introduced the new Speedline filler .
Around 1940, some Oversize bodies were fitted with later broad-band caps.
Collectors refer to these rare pieces as Overmax.
There were also a rare version with the features opposite, three cap bands but with a speedline filler.
Image courtesy of Vacumania.