Belgium? Sg Number And Year Of Issue

Please can someone help to identify the following, I have checked and cross-checked my SG catalogues but can't seem to find this stamp.

All help appreciated
Thanks in advance
sd1964

It is under "Parcel Post" at the back of the Belgium listings. Catalogue no is B527, issued 1929.

Thanks for your help couldn't see it for staring

What you had is "catalogue blindness"
a fleeting, but common affliction of Stanley Gibbons Catalogue users.
Even long time collectors suffer from it.

Wait till you desire a stamp that Gibbons decides to place in
a "definitive list", a list 5 years before it was issued.

Gibbons also like to place their appendix at the back of
the huge tomes, so you have to constantly flex the delicate spines.

Your stamp indicates a rectangular railway cancel, if I am not mistaken.