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Thanks. I wasn't paying attention! That did work when I got the syntax right but I'm still having problems withsimilar searches. I want to put a value in the field "extra" to filter records related to different collections which is the value I want to add to the extra field but in testing I can't get that to work nor other searches. So I've used this syntax when I know there are records containing the word primary in the field extra but am not getting a result returned. I've experimented with other fields &filter=ISSN:0032-4000 for example also without success. Would be really grateful for your help.