Counter-marked 10 cents (Ceylon) value

Got this coin, and I am looking for approx value. Does counter-marks in this case increasing or decreasing value?



Additional info:
http://lakdiva.com/coins/token/1941_10c_gpo_cm.html

Hi
DECREASE -  GPO is general post office here in  g.brit
phill

Hmm.. I would like it !

I would like to keep it, because I do not have many marked coins and I like coins covered by interesting historical stories : ) Like this one- coin converted as phone token. Imagine how important it was to have those two coins to make free calls to home- more important as 10 cents : )

I could tempt you with some debased Brazillian coinage I have a large 80 reis marked down to 40, and a 40 to a 20 ?