My understanding is that the arris rails should be cut a little short so that they are pushed fully into one socket then pushed into place and slid into the other socket, then the end boards are fixed into place to stop them from slipping out.
Perhaps the posts were fitted with the new rails and so just pushed on to a full length arris rail and cemented in.
But I 'aint an expert and stand ready to be corrected!
Thanks for your reply. Another suggestion I've had is to fix timbers to the concrete posts and the use Arris brackets. I'll probably use the bracket method.
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