Bruce, thanks for that - least now I know there is a possibility not all tactical markings were white. I considered yellow due to how similar in shade it appears to the bridge symbol in some photos:
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Mike, not sure about your first question, but the object attached to the Staghound is a section of an assault ramp (bridge). The 'sergeant's car' for each troop would carry a pair these. More info here:
http://www.12mbdragoons.com/stag04.html
regards,
Jack