LAW79 - Sorry to take a bit to get a response in here for you. Was out most of the weekend at the big Springfield, MO Swap Meet and had basically no ability to get to my resources.
The original starter for your '57 6 cylinder application is GM# 1107634 (actually used on '55-'59 stomp starters equipped trucks) and otherwise known in Delco Remy documentation as a "Cranking Motor Group 53-H" unit.
Below are screen shots from a 1969 Delco Remy detailed parts listing for your info and reference that covers all the details of the "53-H" starter. Just find your part# 1107634 and read across the columns to find the related component parts.
Also included are a couple of Chevy Master Parts Book screenshots showing additional info.
The correct starter drive is listed as GM# 1931585 (Delco Remy D-2002) and there are several NOS units for various prices on eBay now,
like this NOS one.
The armature is listed in the Chevy Master Parts Books as GM# 1926626 , 11 - 13/16" O.L. (with a GM# 1935851 as a service replacement in my Delco Remy reference). You can find both
NOS GM# 1926626 and
New Armatures for Delco Starter 1926626 1935851 readily available.
On the "440" casting you've posted, I could not confirm or deny if it's the right one for your starter, but if it has worked to this point, I wouldn't have any concern if I were you. There was one reference in a Chevy Master Parts Book that referenced a "cast 700" for the 1955-57 starter drive nose, but other listings for the part eliminated the "cast 700" when described in other books.
Hopefully these references will help you get what you need to get your starter back to good as new.
