The M stands for "Militare", "Matta" (=crazy) it's a semi-official nickname, a bit like "Duetto"
https://forum.clubalfa.it/threads/piu-matta-della-matta-alfa-romeo-1900-m-matta-ar51-e-ar52.13815/page-2

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

For Russian models they are not always "C" then - but as you said it is a check to do one by one.

- Yes, I think so, since American ones didn't have C as part of the model name
- Yes, personally I haven't (yet?) met in my research an EU model not built in Austria, haven't checked RHD/UK ones though because they don't pull the VIN, but I think we can do all "made for EU" ones made in Austria by default unless proven otherwise?
- Russian ones very often are grey import NA models, so I think those are to check one by one to split EU from NA models.

There is some things to clean in the 300 C
- Those listed as "300 C" instead of just 300 as model should have a "made for" tag
- Those listed as "made for EU" should all be "Made in Austria" I suppose (including those from the point above if the "made for" happens to be EU, or another country for which they were made in Austria)
Is https://www.imcdb.org/vehicle.php?id=843835 an error or where there EU models made elsewhere?
- What about Russian ones? They seem to often be listed as "Made in Canada" but with the 300 C name; they should have a "made for RUS" tag then.

[LE] is the code also for the Chrysler 300C Touring that were built in Austria. Thanks in advance!

For all "Harley-Davidson Iron 883", model is "Sportster Iron 883".
http://www.imcdb.org/vehicles_make-Harley-Davidson_model-Iron+883.html
http://www.imcdb.org/vehicles_make-Harley-Davidson_model-Sportster+Iron+883.html

Ok, I'll do that. Thanks and sorry for the confusion

Updated
For the older ones, alternatively, for batch update of a list of vehicles that have to be picked one by one you can also post links like JB did above, it is easy for me to extract vehicles ID from that for doing it on the server directly.

It's OK, I'm commenting to the one that didn't have the Austria tag
Sorry, 2001 ones should be [RG], not [GS]. These ones: http://www.imcdb.org/search.php?resultsStyle=asList&sortBy=0&yearFrom=2001&yearTo=- 2001&makeMatch=2&make=Chrysler+&modelMatch=1&model=Voyager&modelInclModel=on&modelInclChassis=- on&mk=&origin=&madein=AT&madefor=&role=
Among them, there's a couple that are the older body style, I'll comment on those separately