The concept of a "Type SE" seems to be a Japanese thing: the [canonical list of plug types][1] is maintained by the International Electrotechnical Commission, and they don't recognize it.

The best source I could find, then, is [this random Japanese site][2], which claims that Type SE is identical to the Type C, except that:

 - Type SE plugs have may have a hole for a ground (earth) pin in the middle
 - Type SE plugs may have slightly wider pins than Type C

On closer observation, it thus looks like **"Type SE" is identical to what is usually called "Type E"**!  [Type E][3], which originates from France, has the grounding pin hole and straight 4.8x19 mm pins, while Type C lacks the hole and has slightly smaller 4x19mm pins.  Side by side:

[![enter image description here][4]][4]Type C 
[![enter image description here][5]][5]Type E/SE


  [1]: http://iec.ch/worldplugs
  [2]: http://www.traveltowns.jp/electricity/type-se/
  [3]: https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#CEE_7.2F5_and_CEE_7.2F6_.28French.3B_Type_E.29
  [4]: https://i.sstatic.net/B7PSP.jpg
  [5]: https://i.sstatic.net/HSEqM.jpg