General Technical Questions and Answers last answered over 1 year ago.
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If it's an Aircooled Devon then Im fairly sure they Combi based and already have seatbelt mountings.
Does it have factory windows or is it a converted panel van?
Then it has seatbelt mounts.
Look under the back seat for what look like plastic screws with a philips head, inscrew them to find the threads already there for seatbelts.