It looks like the brushes at the top in the photo don't have much spring load when you compare them to the lower pair.. lower pair closed up top pair hanging wide open. Both sets of brushes should be fairly tight together if the spring loading is good.

What does this starter go to? (type of machine) Depending on the battery size you probably wouldn't see a large drop in voltage. I do see in your picture that the brushes do have a spring mechanism in the arm. So they would be contacting properly. There is a high resistance somewhere. The only thing I can think of is to assemble the motor, attach the meter to the leads with clamps if possible (if not strip a section of wire and wrap it around the meter leads so you don't have to hold it. ) Set meter to low ohm reading. Rotate the shaft SLOWLY I mean one full rotation should take you 30-60 seconds. see if you get jumps in ohm readings. It will jump around a bit but when done slowly, you will not get an inductive EMF backlash voltage reading. (from the magnets) It is possible the brushes are not maintaining contact as it rotates. I would feel the shaft with the commutator and feel for any bumps, that would deflect the brush for a moment and lose contact. Another test is with the starter assembled, connect one lead to either connection, the other to the frame, set meter to 100k range and slowly rotate the shaft. You were getting a static reading of about 10K which isn't good. If it changes a lot that could mean the power is bypassing the windings and the armature is no good. What is the highest DC amp range your meter can do? I would re-check the windings. Put a meter lead on one commutator point, the other lead to the one next to it. do this all around the armature. Ohm readings should be the same. At 0.1 ohms, that starter could pull 120 amps from the battery under full load.

Good Morning Chinese New Year! Goodbye year of the Snake. It's the year of the Fire Horse! My friend says a Fire Horse is a Stallion...LOL So guys take heart and get your spurs out and Bridle Ready. It's the year of the FIRE HORSE!