sorry i'm late to this thread,
the 6V starter motor should max out at about 525 amps, that's the locked test specification, see attached excerpt.

any rotating current will be less than locked (universal for all rotating machines)
once rotating there is back EMF, locked scenario has no back EMF to oppose current.

the no-load test should be 65 amps for 47-53 6V starters

to test i recommend:
clamp the starter body in a large vise,
attached amp-meter in series with the positive post of the batter and the starter,
ensure the amp-meter polarity is correct for anticipated current flow
touch negative of the battery to vise by hand
do not run it for more than about 10-15 seconds no load, it's bad for the commutator
once the RPMs have settled down and running smooth, note the amp reading then stop the test

i would use a clamp DC meter for this application
set it to the 1000 DC amp range
-s

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