I usually jack on that vertical mounting plate, myself. Many posts will tell you to never jack on the axle. The location of the "jack here" stickers on my Caravel
were just aft of the mounting plate, directly under the frame.
Some folks think you will bend the frame, others are concerned that the thin edge of the mounting plate makes it more likely to somehow scoot off the jack. I think either the frame right aft of the mounting plate or the moutning plate itself are fine, just watch out for interference of tire/wheel as you jack.
Stay off the axle.