The Stephens pen with its push knob style filler is an elegant version of the button filler. Unscrew the end of the barrel. It appears to be a blind cap, but will only unscrew part way. Then this cap is pushed to press on a standard pressure bar. Laurence Oldfield has a very thorough write up on this pen with far more details (patents, disassembled mechanism, etc). StephensTechArticle (penpractice.com)
