[First of all, I'm a big fan of using the original script of languages, but seeing as Arabic can do some crazy stuff with displaying properly on computers I'll use transcription only today]
Classical Arabic has approximately 6 types of noun endings, which can be homophonous in some forms, but in the most basic paradigm there's 6 different forms.
For example the word man 'rajul'
As you can see, when a noun is indefinite it is marked with an additional /n/, this is so called nunation. This goes way back to proto-semitic, where it was still an /m/, and thus apporpriately called mimation. Although in fully vocalised texts this nunation is incredibly clear, when we just look at the plain, unvocalised texts, it becomes a lot weirder.
Where's the nunation now? It disappeared! In unvocalised writing, there's actually no proof that there is such a thing as nunation. And the spelling RJLA seems to indicate a word rajulaa (which incidentally was the form in Old Arabic, or so I've been told).
And although you could say 'that's simply a spelling convention', this is in fact very odd. Yes these days it's a spelling convention, but as some of you may know, Arabic exclusively writes consonants and long vowel. I think we can all agree that in fact this nunation represents consonants. Why then isn't the indefinite paradigm like this?:
acc RJLA (RLJN?)
I personally believe that the written Arabic doesn't actually represent the Classical Arabic language, but rather a later stage that already lost its nunation (like all following Arabic languages have).
So the people who first wrote the Arabic language had already lost this nunation, but I believe that there must have been some religious tradition that continued this grammatical structure, which had been long lost when the Arabs started writing.
This is not entirely without precedent, think of the Sanskrit Rig Veda, which retains a much more archaic form of Sanskrit than we find in other texts. Not because it had been written down earlier, it was written down around the same time as classical texts. But it was passed on orally for much longer than the other texts, with virtually no change to the original position, thus retaining arachaisms not seen anywhere else.
The situation isn't 100% the same, as the Rig Veda texts, indeed retained old archaisms, but the language was never used to compose new texts. Which seems to be the case in Arabic.
I know that this idea feels 'far out', but Arabic combines fantastically with the other Semitic languages like this. Hebrew for example also lost its nunation (and case endings all together), but has some vestiges of this /aa/ from the Accusative indefinite.
Take layla evening. This is, although it looks just like a feminine, in fact a masculine word. the -a at the end is actually a retention of the Accusative indefinite long aa. Denoting 'during the night'. This is the exact same form that you see in writing of Arabic LYLA.
Although this all seems so crazy, I can't think of a good reason why nunation would not be written. Recognising that this nunation was a special grammatical ending, is way to linguistically deep to realise for the people who started writing it. That would be like Sanskrit using an s rather than an ḥ to denote the word final s, but then actually quite a bit more extreme.
I've looked for Old Arabic sources, hoping to find some nunation there, but even in those text it seems to be absent (although it's incredibly hard to tell).
One explantion often seen as to why nunation isn't written, and why the ending -an is written as -aa, is that written arabic is written 'pausal' The way you'd write the word in an individual context. This also explains the unpronounced Alif al-Waṣl (Connextive alif). I don't buy it though. In earlier texts for example, the Alif al-Waṣl was not actually written, because in many contexts it was unpronounced. So if back then they didn't write pausally, why didn't they write nunation?
I don't need any of you to agree with this idea, but what I'd like to hear from readers with at least some knowledge of Semitic, is a plausible explanation why nunation wasn't written.