This is my take in the discussion on photo-3d about refurbishing old stereo cards. The examples are downloaded from https://www.loc.gov/photos/?q=keystone+view+company (Congress Library). They are aligned and rendered generating two separate left & right aligned images using STMANI3. Then mixed the aligned images with the unaligned raw scan of the card using GIMP (this step was the most time consuming)

https://www.loc.gov/item/2013649296/