Good luck with finding one.
I have been researching and collecting Industrial ARP badges for over forty years and have never recorded a definite Austin Motors example. Given the size of the company it seems strange that one has never surfaced.
The attached picture of an Austin Motors wartime service certificate however illustrates on the right a series of what appear to be a company ARP badge which does not carry the name Austin but does have different coloured backgrounds, a common way of indicating the different types of ARP service.
If you do come up with one please show it on the Forum.
Keep your eyes on
http://homefrontcollection.com/ who come up with some excellent Home front items.
Jon