@PW: the LFddnn are not limited to microlight fields, for one example the Verchocq Airpark is not limited to microlights. But yes, microlight fields were the prime content and still are the bulk.
@PW: [[ Are you sure you haven't found some virtual airfields? ]] No I am not sure. At the contrary some may well have fallen into disuse. I found them on openstreetmap which is less than encyclopedical.
@Katamarino [[I'm pretty sure only the USA has more than 26x26 airfields]] : I am pretty sure you are mistaken there. Think Russia for one example, and the UK as already pointed out. France also has some 800-900 listed, and probably many more unlisted.
@ninja-lewis: Thanks, that looks like the best answer. The chap explains how he came to conceive these codes, it seems to be well-thought. Only sad the excellent initiative was not continued.