On an India Registered cover I have ;the cross on the cover is
in blue.In Ceylon the green markings are perhaps for postal revenue
Wow! a beautiful image of the 50 rupee, Perf 12 !
I looked everywhere for one.
Wax Registration markings are common in Red or Blue, particularly in Commonwealth countries.
Green I have never seen, also, with the wax lines not intersecting at right angles, I remain with the opinion they are random re-use prevention.
Postal Revenue usage is a valid option, that is beyond my knowledge
I have a Ceylon booklet somewhere in my Library, unable to locate it today.