As it turns out, those numbers appear to be the cabinet maker's serial number. (We have not seen 2 RCAs with duplicate numbers under the cabinet doors yet, so we're fairly confident in saying this). The real serial number for the theremin itself, 100115, is stamped inside the cabinet.
Buck has also shared some new photos with us, including one of a custom case made for the theremin.
See Buck's listing in the RCA Registry for more details.