King Albert II 's monogram is on the Belgian euro coins (A with a crown above). Albert I's monogram was probably the same as Albert II's.
If I am not mistaken,the Leopolds' monogram was an L, mirrored to the left side, with a crown above (so two L's, the first mirrored).
Boudewijn's monogram was probably just a B with a crown above. Maybe mirrored as well, can't remember, I was only a child when he died.
I hope this helps you somewhat. Haven't found pictures, but euro-coins you can find on Wiki. Maybe pictures of royal buildings can help, I distinctly remember seeing a Leopold monogram in greyish stone one day.