It appears that the first three D-16s converted by Riley Aircraft did not receive a Twin Navion (or TN) serial number. Its known that the original Twin Navion conversions pre-dated the CAA's Supplimental Type Certificate (STC) process and were originally certified under Note 3 of the single Navion's Type Certificate. Because of this, all D-16s had to carry both their original single Navion and new Twin Navion data plates.

Based on research done by TwinNavion.com (both through the FAA records, and by inspecting the two surviving aircraft), its our belief that the first three D-16s built by Riley did not receive serial numbers in the TN series, but maintained those serial numbers assigned when they were single Navions.

NAV-4-1832	NAV-4-65	NAV-4-680	TN-4
TN-5	TN-6	TN-7	TN-8
TN-9	TN-10	TN-11	TN-12
TN-13	TN-14	TN-15	TN-16
TN-17