There's no updates of SRD-files during a RMU download for City Navigator NT. The RMU package may contain one updated SRX-file and several updated ASR-files.
SRD-files are handled by WebUpdater which scans the "GarminDevice.xml" file to find out what needs to be updated. I've made a test with a USB stick and a copy of a nüvi 3790T directory structure. Just some small editing in the GarminDevice.xml makes WebUpdater to suggest updated SRD-files.
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