With the Vacuum Filler Parker introduced both a Top Line and Bottom Line series. This distinction continued with the Vacumatic.
The main differences were:
There were four primary sizes in the Top Line series.
Click the pen that correlates with yours in size
From left: Oversize, Slender Oversize, Senior, Standard and Slender. The Slender Oversize replaced the unevenly ringed Senior in 1936 and was later discontinued for the Slender Maxima.
Note: Production sizes may vary slightly.