Bobbee's on to something....His 1922 ad (again below). For the $30 Octagonal- is the text describing and graphic depicting an engraved case? Am I seeing the wording for the octagonal LM saying "...solid white gold engraved case-dial.." or am I seeing things?
Looking at the $30 watch below the octagonal example, the text says "...Solid 14kt. engraved white gold case...".
The 1921 ad bobbee posted has an $18 round and a $20 octaganal LM which appears to be smooth (not engraved). The text in this second ad gives no indication of engraving, but price and text (25 year octagon case) suggests gold content of these "cheaper" Lady Maxims".
Based on this these 1922 and 1921 ads.
(engraved??) Octagonal $30 solid gold Lady Maxim
(non-engraved??) Octagonal $20 gold filled Lady Maxim.