Alfa Mattas were officially either AR51 or 52 'Matta's. People often stick '1900' before that because of chassis plate saying 'Tipo 1900' even though this is not found in documents. But 1900 M Matta doesn't make any sense since M is abbreviation for Matta. So it's one or the other. I didn't find anything to explain what 'M' might mean other than Matta.. Plus in period brochures I see the full 'Matta' designation with quotations. Above comments can also be found on the first of the following cars. AR51 or AR52 can stay as description wherever available though there really is no way to tell (even indicator position etc don't say much, the same for civilian 52 versus military/police 51 -they can be either). So...
These should be 1951 1900 'Matta':
www.imcdb.org/vehicle_1145685.html
www.imcdb.org/vehicle.php?id=209037
www.imcdb.org/vehicle.php?id=327398
www.imcdb.org/vehicle.php?id=548068
www.imcdb.org/vehicle.php?id=995813
www.imcdb.org/vehicle.php?id=1456209

..and these 1900 'Matta' with whatever year:
www.imcdb.org/vehicle.php?id=167768
www.imcdb.org/vehicle.php?id=883141
www.imcdb.org/vehicle.php?id=1146462
www.imcdb.org/vehicle.php?id=1149015