We previously used a Bulldog Euro on our first three Eriba's and were happy with it. Over the years this has had several model numbers but all are basically the same. We swapped to a Nemesis Ulta on our previous van as this is lighter, easier to fit and we have stuck with it. Both are available in excellent condition second hand on the likes of Gumtree, Facebook marketplace, e-bay etc and this is how we bought ours. Loads up for £50-60 https://www.gumtree.com/p/campervan-cara...ock-/1498636655
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As regards your hitch lock. I would advise the genuine Alko lock and not a third party make, no matter how good they may be. They are more compact and you can tow with them in place. This later point is a contentious issue as some say the emergency services do not like you doing this.

Colin

EDIT - following Simboc's comment below

If your 535 is a new, post 2024 on the Knott chassis, then it will have a Knott hitch and will need a different hitch lock
If your 535 is pre 2024 it should have an Alko chassis and Alko hitch, so go with the genuine Alkpo hitch lock.

I believe Eriba took the opportunity of switching to a cheaper option when they dropped buying in Alko Chassis. As we know, cheaper is not normally better.

Colin