Rambam (1135-1204) was at that time in Cairo and he was able to view it and endorse it as authoritative. He writes that he consulted this very text and considered it to be the most accurate and authoritative Torah text against which all other texts could be proofread. Rambam wrote; “This was used as the standard text for the correction of Torah scrolls. Everyone relied on it as it had been corrected by Ben Asher himself. And I have relied on it for the Torah scroll that I wrote.”
|Ezra Dabach, one of the last key holders of the Aleppo Codex|
|Ha'azinu Song in the Aleppo Codex with 67 lines (Note the vocalizations)|
Another tradition recorded in Midrash Talpiot, puts the number is 310 674.
It should be pointed out that there are different systems of counting Hebrew letters because sometimes two letters can together form on single letter. The large number of 600 000 is most likely allegoric.
UPDATE: (1 Jan 2017)
WAS AHARON BEN ASHER A KARAITE?
I was always bothered why the Master Copy of Ben Asher was placed in the Karaite synagogue in Jerusalem at the beginning of the 11th Century. I was astounded to discover that there is much debate over whether or not Ben Asher may in fact have been a Karaite himself.
It appears as if, from Cairo Geniza fragments, the ben Asher family may have been Karaites for a number of generations. This may have been why they were so preoccupied with the preservation of accurate texts.
Rav Saadia Gaon, known for his fierce debates with the Karaites, referred rather critically to a 'Ben Asher', but no one connected him to the famed Ben Asher of the later to be known Aleppo Codex - until the relatively recently discoveries in the Cairo Geniza.
On the other hand, Rambam (who also debated - but more amicably - with the Karaites) was known to have had quite good relations with the Karaite community, and we know that he endorsed Ben Asher's Torah.
A quote from Aharon ben Asher's Sefer Dikdukei haTe'amim reads: "The prophets...complete the Torah, are as the Torah, and we decide Law from them as we do from the Torah."
This sentence, may previously have been overlooked as an innocuous statement, but with apparent evidence pointing in another direction, it may support the notion that he may have been a Karaite, as they certainly do 'decide law from the Torah'.
This is not an entirely compelling argument as the Rabbinites also base their Law on the Torah, although with more latitude. But it does make for fascinating study to think that our (third) Master Copy, after Moshe and Ezra, came from Aharon ben Asher who may have been a Karaite!