CDRs aren’t categorized or handled by extension numbers because in Asterisk an extension is anything in the diaplan that can be matched.
You would need to know the unique ID of a CDR to get that single CDR. In order to get that you need to pull all the records to know what IDs are what. You can limit the CDR grab by time/date so you don’t have to pull all the CDRs to get the CDRs over the last week, for example. You can get the CDRs for the last week and then get the one with the recording you’re looking for.