No.1 Cardboard or plastic mount no glass is better with a curved field lens.
No.2 Slides in glass calls for a flat field lens.

The major difference of a Leitz projector over a Kodak or other branded projector is the lighting system.
EK and the others use a heat absorbing glass, to protect the slide. Leitz does not use a heat absorbing glass
but a Dichroix [SP] reflector to absorb the heat this results in a Whiter light on the screen.
This can be shown this way. Set up two projectors a Leitz and a Kodak side by side to project on a large White screen.
Use slide frames to open the mechanism and adjust the projectors to get the identical size os on the screen.
The Leitz will be a pure White while the EK will have a Greenish tinge.
I'm sure this will have some effect on a slide projection quality.