This new thumbrest from ProTec features an extension plate that runs down the back of the instrument, under the thumbrest. This is in place to help support your thumb not just from the padded rest but also as it touches the back of your instrument. It may also help prevent slipping when you've been playing for some time.
This is the SMALL size version which is best suited to narrower clarinet or oboe thumbrests, smaller than 13mm diameter.