Today I installed RAM Mounts Large Marine Electronics Mount RAM-D-111U with Medium Arm. I noticed inside the arm there are positioning post for springs on each end. My unit has one spring installed on the lower base side. Should my Medium arm have 2 inside tension springs? Is there a part number I can order if needed?

There is only one spring in the double socket arms like RAM-D-201U-MI1.  That allows you to loosen the arm and only have one side open enough to remove the ball, that way the side you select (the side opposite the spring) will stay tight and either keep the arm clamped onto the ball base that's serving as the mount, or the side that has the marine mounting plate (in your case) so you can remove the arm as well as the marine adapter plate from the base.

What is a Jam nut?

The jam nut is an additional nut that is inserted over the threads of the socket arm bolt that literally "jams" against the threads of the bolt, preventing the socket arm to be loosened to any degree to be configured by the user. With this jam nut in place, the knob cannot be loosened to a certain point, without the use of an additional socket wrench tool to remove the jam nut.