Ok...
A standard RM2 will NOT fit. The catch cup is too small. You need to ask Rob to mill a RM2 down to size to fit a 14500 mini.
A 2.1 uses an 18490 battery instead. Consequently it's a bit larger in depth ("fatter") to accommodate the larger diameter battery.
You can "go there" and get a RM2 for a 14500 mini, but be aware of the limitations of those batteries. You need to stick to pretty high resistance builds. There's only so far you can go with that battery capacity.