The stitching on the majors crowns is pretty poor and might not be original to the smock. You do also sometimes see generic khaki backed rank insignia, though as you mention standard para would be light blue backed. Interesting with the X on the back of the collar.
Khaki backed rank insignia was very common on field clothing in the 1970s and the sewing was often done by the user, so may not be that good. The white X on the back of the collar is for station keeping on night patrols etc. It is more often seen as a white rectangle and is still used by old sweats....Nice smock.