St. Martin was a depencency of Guadeloupe until 2007, and thus used department code 971. Until 1979, numbers on this island deviated from the normal French system by having the code as a prefix. In 1979, when serial numbers where in the 3000s, 971 became a suffix, getting the island in line with the rest of France.
After 9999 SM was reached, the letters ZA and ZZ were used, then three letter codes from ZAA onwards. With the introduction of the Euroband, the digits 97 often became stacked to save space.
In 2007 St. Martin split from the overseas department of Gouadeloupe and became an overseas collectivity of its own. It was assigned code 978, but so far has not implemented that on the plates. Because it became an overseas collectivity (not a department), it did not change to the uniform French numbering system in 2009.