There is a thread on SCF that explains these.
edit: The below is wrong information. A guess, going by memory. Please ignore the remainder of this entry.I believe it is a delivery postman's personal cancel. Each area (of Paris, or France?) has it's own special shape (triangle, circle, hexagon, etc) and each postman had his own letter or numeral, sometimes Roman Numerals).
This is a, most likely, bogus cancel, pic from
ebay France, showing one allegedly from Paris 6th arondissement, No 3 postman (I think).

I will try to find that thread and post a link. I may be barking up the wrong tree also.
If that was a T in the triangle I would say it was for Taxe or Postage Due.