Here are a couple of different plates.
These cast housings are flat on both sides.
The starter plates both mount the starter flat as well.
I think, could be wrong, but the stamped bellhousing is flat to the engine but moves away from the engine for the starter plate. That requires a plate where the starter mounting point is at an angle to straighten the starter with the back of the engine.
I'm sure Mac is right about there being more designs; this is merely what I have. I hope others will add their wisdom.