My knowlegde on the older 2CV's are not so good, yet. So I'm into learning about them.

I have two 425ccm 18hp engines. One of them, marked AZM, has openings in the top cooling ducts. The other one, marked AZ, doesnt. See picture where my glove points.



I'm so curious about what the opening are for....
And what type of A-model did the AZM engine live in?

Maybe a carb pre-heating for cold countries? The first 9hp had also some type of pre-heating. See picture a welded version on the manifold and later a bolt on version.

425cc (1963-1970)
Type A53 - Plaque moteur AZA, AZC, AZM
16ch Din à 4200trs/min - 27Nm à 2500trs/min - Modèles AZ (3/63-2/70) et AZU (1/63-8/67)
Quatre vis de chaque côtés du collecteur
Tôles de refroidissement des cylindres plates
Dynamo sur le vilebrequin (6V)
Ventilateur à 6 pâles en acier
Grille de ventilation fixée par trois vis (une seule en bas)
Ancienne boîte de vitesse
Carburateur Solex 28BCI

Pre heating from the manifold makes sense I guess. But from the sylinder?
I suddenly came to think of those German models with the extra tinware around the top of the cylinder, against the bottom of the cylinder head. Apparently to guide away exhaust fumes from the air blowing into the cabin, in the case of a loose head blowing.
Maybe I'm way off, but might this be something similar? I haven't dismantled the cooling ducts, but this tiny opening seems to be some sheet metal mounted to the top of the sylinder, against the head, inside the cooling ducts.
Some sort of health and safety thing?

Don't think so. I also showed your picture on the french citroen truck site and they think it will be for an pre-heater of the carburettor. In my 1967 2cv parts guide there are 2 pages for parts for -15 degrees heating. But helas without drawings. There was also an auxillary petrol heater available. Could it be for that?
https://www.2cv.fi/extra-heating-2cv-heat-exchanger/