The ML with the crown is the maker's mark.
From my information:

MANUFACTURE LIEGEOISE D’ARMES À FEU
Rue du Vertbois, 54 à LIEGE
1866-1929

The ECM3 is the 4th proof test. They are testing with smokeless powder, unlike the first 3 tests. There were a number of smokeless powders available on the Belgian market. One of them was M. Some others included L, T, S, etc. Later this powder designation was dropped because of the confusion it caused with hunters. There was also a so-called 5th proof test for a period of time.

Who determined the type (maker) of the powder is unclear. It could have been the Controller of Proof (inspector) or the gun's maker.

The proof pressures would have been around 14,000 psi for a 12 gauge.

A really nice gun at a great price!

Pete

http://www.damascus-barrels.com/Movie.html