Well, does not appear to be the fuel filter/pressure regulator.
A correction to my first post on this thread. It was not a WIX filter, but a made in Germany Mann Filter WK 720 that was installed to replace the Chrysler/MB original unit. The second replacement was a MB filter, number A 002 477 30 01 actually made in Germany. Installing the new filter made no difference in the cranking issue, therefore the check valve in the filter/regulator does not appear to be the problem. Oetiker clamps were again used.

I've attached pictures which are of the filter/regulator and hoses. On photo 2375, there is metal canister installed inline - this is outlined in a red circle. It is inline with the fuel line going from the filter/regulator to the fuel rail/engine. Any idea what this is?

Another attachment, labeled fuel system schematic, is from page 14-2 (volume 2 of 3) of the factory shop manual. Note that this schematic does NOT show this gizmo. Alternately, page 14-10 is another drawing which does show the thing but does not identify it.

Reading elsewhere in the manual, it suggests 1 of three potential problems. 1) check valve in filter/regulator is bad - changed it already, so probably not it. 2) check valve in fuel pump is bad 3) leak in lines 4) leak in one or more injectors. Items 2,3 & 4 require testing pressure at the test port on the fuel rail - Snap on dude comes Tuesday mornings so I will get the correct adapter to fit my tester so I can do more tests.

Besides waiting on Snap On, anyone have an idea what the thing inside the red circle is?

Should have left the dang thing alone!