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I do not recommend continuing with that starter. The armature shaft has been modified and who knows what else.
BUT:

There is a step/shoulder on a 1867897. This is a transition to a smaller diameter on the nose portion of the stock shaft. The stock spacer/thrust washer goes on that smaller diameter and that smaller diameter fits into the nose bushing. That is what keeps the shaft retained from moving, and at the commutator end it's retained by the end plate. Shaft is supposed to be sandwiched between nose and end plate. The retaining ring shown in the diagram is for the clutch assembly. Does not attach to the armature shaft as you know. There are no retaining ring grooves on 1867897.

It seems your shaft was shortened and lost the shoulder and the smaller diameter section of the nose end of shaft. So they added a retaining ring groove and a retaining ring for the spacer/thrust washer to bear against and act as a shoulder to retain the shaft. But then must have modified the nose hole/bushing to receive the larger shaft. Then another person added washers. But as you say, the washers keep the pinion gear from extending into the ring gear... or something does. You need to figure out why the pinion gear travel is less with switch and levers on. Several issues to get right.

1867897 was superseded by 1917855. both obsolete but available on Ebay, etc. because used in so many starters including John Deere.

I guess we have to assume your shaft was shortened to fit into a 12V starter from somewhere. Don't know.]

You can tell whether an early Chevrolet or GMC starter is 6 volt or 139 ring gear teeth because the starter drive pulls out straight. The armature has straight splines. The 12 volt or 168 tooth ring gear starters have helical splines on the armature and will twist when pulled out.
Also the "exposed end of the starter shaft on the 12V starter will be approx. .470 while the 6V starter has a shaft diameter of closer to 5/8". Maybe that's how the chopped shaft fits in the nose.

There is a phenolic thrust washer on the commutator end that could be worn or missing. Maybe that could cause movement.

I don't know if Ken is still in business, hasn't posted for a long time. But he will help you with any starter questions, parts and rebuild. Really good guy. classicgenerator.com
Email him from the website.
On Stovebolt he is kennethw