The Conway Stewart 300 was one of the first lever fillers made by the company in 1919. this version, featuring a locking paddle lever with lug attachment, is from the mid 1920s.
The pen is constructed in Black Chased Hard Rubber (BCHR), there is some brown patina, I have made no attempt to change it as it is really part of the aging process. The surface is good and has a nice finish, the imprint is worn but the 'No. 300' is just legible.
The pen features a 'self locking lever', in this case a 'paddle' style, which locks flat when to prevent snagging on clothing when placed in a pocket. It was an apparently simple innovation that no doubt prevented catastrophic damage to countless shirts and Jackets.
At 13.7 cm. capped and a huge 18cm. when posted, it is a very long, elegant pen. Made at a time when a pen was an essential item, to be used for extended periods of time the posted length and very light weight of 11.5g. were desirable attributes. Pen adverts often focussed on the comfortable feel and effortless, fatigue free, and long lasting considerations rather than appearance.