The optics of this Nuremberg microscope consist of only two lenses: A: objective (dia=1.1cm, f=1.5cm) and B: Eyelens (dia=1.9cm, f=6.5cm). There is no Field lens and no obvious place to install it.
This lens was found tapped into the eyepiece dustcap. It is a bi-convex lens (dia=1.4cm, f=4.2cm) in a metal mounting frames. It probably is not original to this instrument.
There is only one objective. It is a bi-convex lens set in a depression in the wooden mount.
The dustcover screws onto the distal end of the eyepiece.