gas flows from ismay production interval..., page-13

There is no universally agreed symbol but the following are used:

cubic feet, cubic foot, cubic ft
cu ft, cu feet, cu foot
ft³, feet³, foot³
feet^3, foot^3, ft^3
feet/-3, foot/-3, ft/-3
CCF for 100 cubic feet (C denotes centum, hundred. Used in the billing of natural gas and water delivered to households.)
MCF for 1,000 cubic feet (M denotes mil, thousand)
MMCF for 1,000,000 cubic feet
BCF for 1,000,000,000 cubic feet

as you can see, M stands for 1000. So MCF means 1000 cubic feet.
(Wikipedia definition)