Vernon, thanks for your replies. but my truck is an AAR Type A, your examples are a Type B.
SWRR thank you, the sideframe is the Type A. relevant info are the attachments points as shown.
Joe, I looked through all available truck part examples on a MTH site I found. did not see any match. don't know if their list is comprehensive.
who else had removable bottom plates in a 'china drive' ?