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Summary & questions:
1- stamps are COIL stamps printed in recess (+ photogravure)...
Yes and no... Michel does have a category for intaglio recess (StTdr/Stichtiefdruck), but that is not the same as WIFAG recess in my humble opinion (Rein, please correct if I'm wrong).
Like with perforations, colors etc. catalogs (try to) simplify things. Instead of 'facts' you get 'altered version of truth'.
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2- numbers on the back are:
They're counting number, should be for every 5th stamp.... The page Rein linked states the coil machine provided stamps in strips-of-five.
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3- which perforation reference is the correct one, so to compare my stamps with?
None... Catalogs always round the results.
If you want precise figures, then try this:
http://www.stampcollectingblog.com/...ation-gauge/ (there are several alternative apps/tools to measure perfs, but since I built this, I know it works flawlessly, LOL)
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4- does STRIPs of 2,3 or 4 have got (btw, I couldn't find even a STRIP of two on the internet) a different catalog# & catalog value?
Not in general catalogs. Likely country specific special catalog (in this case POFIS) would provide reference to complete strips (of five).
And like I wrote, what you have is not rare. Have seen several items such as the ones you've got over the years/decades. The reason you don't see pictures of them is that most (western) collectors consider them as 'starter packet material' not worth retailing individually. And to certain level I agree with the assessment. Here's couple of random coil pairs I pulled from my duplicates box after some digging:

So the fact that you don't find images of pairs and larger strips using google does not mean they do not exist.
-k-