Belgium Overprinted "T" Stamps

Some Post Offices who hadn't Postage Due Stamps in their officed used the stamps from the emission of 1915 with a hand stamped T to receive the port.

After 1958 the use of the T-handstamp became regular.  

For the stamps before 1958 it is preferable to collect these handstamped items on cover.  Only on a dated cover one can be sure it was really used as Postage Due.