There a few other resources on carbine buttplate identification, right or wrong. There are some photos on the Chestnut Ridge Supply website: Chestnut Ridge Supply Gun Parts
There are photos from the Carbine Club newsletters in Larry Ruth's "War Baby" book.
If you happen to be a member of the Carbine Club there are photos in the Newsletters.
It took me a while to figure it out and there are certain buttplates that are still is question as to which manufacturer they should be attributed to. It gets real challenging when the buttplates are worn or dented.