Hello Spivey, if you had called them splashers you might have had a reply sooner!
They are the oil boxes for the axleboxes and possibly for the hornguides as well on the full size. If you make them to the drawing they will only be for decoration, too small to hold much oil. There will be little pipes leading down to a suitable point on the frames.
Interesting that people still want to build Maisies 75 years since the design was published and 54 years since the last 'Atlantic' ran in Britain. Still a really good loco though.
All the best
Nick