Sounds about right for 9x12 holders you'll find the metric dimensions for a few in this section of the Forum.

You can easily find 9x12 holders and even roll film backs on Ebay, but being sure they'll fit an Avus is partially luck, and how good the photographs & descriptions are. Search Ebay.co.uk and Ebay.de (login details are identical as all are on the same US server).

Welcome to APUG BTW

Ian