If the starter is an original, the number (and the date it was made) are stamped on the round armature case, about an inch forward of the joint between the case and the starter nose. If it's a rebuilt, the number is probably meaningless, as rebuilts are assembled from bins of parts from stripped starters, and the armature case could be from anything.
You need the starter brace that goes from the 1/4"-20 stud on the forward end of the armature case to the 5/16"-18 tapped hole in the block to support the forward edge of the starter; without the brace, the starter will be noisy and the drive gear can bind on the ring gear.
There should be about .030" gap between the end of a tooth on the starter drive gear and the root between teeth on the ring gear with the drive gear engaged - about the same as a large paper clip. Shims were rarely used on original installations.