It's the part that incorporates the bottom outer sill as well as some of the inside floor like in the pics below
http://www.rustbuster.nl/Bus_zijpaneel/ ... hzstuk.jpg
http://www.rustbuster.nl/Bus_zijpaneel/ ... rtpunt.jpg
The inner side member sill is obselete
Just thought I'd impart that bit of amazing information
