1x 21700 to 2C kit uses a singe 21700 cell and the factory Maglite tail spring. The sleeve has a long brass screw inside to provide electrical contact and take up the extra space in the body. 

As-is, this body kit is perfect for use  with the Firefly 325 lumen AND the 800 lumen Firefly 3-6 D and C cell LED conversion for Maglite, luminus SST40 emitter.

With the addition of a C-ring adapter, however,  it will convert your C Mag to a 2,500 lumen powerhouse to host the Lumencraft 2,500 lumen bulb!   Sleeve dimension 3.6"(91.49mm)L x 1.03" (26.19mm)W.