Here is an image furnished by Ed Uberall, actually a composite from two different issues of "The Trainmen"... from 1919 and 1922.
As you can see in the left ad, the 'flat bottom' crown.....
In the right image, you see both the flat bottom and the more-often-seen angled crown...
You can also see two styles of dials.......
Thank you Ed for the image...