My new site all about 30 assault unit has many photo's, soon to be more, some if which show vehicles of the unit.
I'm also researching 30AU (for a 1/35 dio).
Attain by Surprise (ISBN 0 9526257 2-5) edited by Sqn Ldr David Nutting MBE RAFVR (one of 30's pet crabs) is well worth a look. There are a couple of pics of Humber's (one named "Sasanach") as well as both armoured and unarmoured jeeps.
Sadly none of the pics are good enough/the right part of the vehicle to show the Tac Signs (although all the ones in this book were crewed by RM rates, not RN) so I would go with the RM formation sign (at a guess).
Sorry I can't help you more.
Please drop me a note if you have found anything else...