One thing I keep forgetting is that its depth adjuster is reversed, i.e. turns clockwise to withdraw the blade. Also, I don't know if it's a stock adjusting knob, but it's quite tiny, and a bit of a pain to get ahold of.
My knowledge on Sargent planes is very little. I do know that Stanley and some others started with right hand threads on the adjuster and then changed to left hand threads. Not sure if Sargent also changed the handedness of their adjusters.
It does look like your adjuster might have been of an earlier two piece design and part of it went missing.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)