Well if your talking the AW IMR thats recommended there is only one .....
Some vendors list it as a 18500 and some 18490 which is whats printed on the red AW label.
The black aw protected battery is an 18500 and it does fit in my 18490 REOs but its to tight for safety IMO.
Plus it seems a touch bigger in dia so it squeezes against the bottle and might not drop away if the spring does compress enough.
Now if your talking other brands which you shouldn't I have no idea what they measure. from past experience the 49 or 50 mm length might vary up or down 3 mm.
So in summary use the
RED AW IMR
batteries either way they are listed for sale.