National stamped the serial numbers of early HROs into the top of the chassis just outside of the antenna connector.
Later HROs including the HRO 5A, 7, 50 and 60 have the serial number stamped into the chassis just behind the crystal filter "can".
Also, all coils aligned to a particular HRO have the HRO serial number ink stamped on them.
For help dating a particular serial numbered HRO go
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