Hello Dave-

The Mahillon name from Belgium is an old and highly respected brand of firearms.
For the most part they were a retailer, but a retailer of high quality equipment.
Many of the world's most famous makers, regarded as the best, also have guns and rifles (even of 'best' grade) made by outsource craftsmen and gunbuilding shops.
If your rifle bears the Mahillon mark, it's a sure bet it's a high quality firearm.
It is typically not very easy to determine exactly who or which shop built any particular high grade Belgian firearm.
Your rifle has special stampings in the metal known as proof marks that help tell the story of when and sometimes by whom it was built and test-fired at the local proof-house. There also may be marks indicating what specific load it was built to perform properly with.

What can you tell us about your rifle?
How long have you had it, what is the configuration (SxS,O/U,sidelock,boxlock,ejector...) and how is the condition?

Can you post images of your rifle?
If you can get us very clear images of all the marks and engraving on the rifle, we may be able to help steer you to an answer with more useful information to you.
The proof marks will be found on the surfaces of the barrels that mate with the rifle's action metal, also there will be marks on the action surfaces that mate with the barrels.
There will also be marks on the barrels further forward of the action flats, under the forend wood.

The more information we have to look at, the better we can help you identify the specifics of your nice double rifle.

--Tinker