Did the 500 not also have this engraving?
Yes, most probably (my T-500 *has* the same engraving - the edging goes a little less deep and is smoother, though). By stating the facts about the engraving I wanted to say that the sax in question is most probably not *older* than the 6 series. The engraving alone doesn't determine the age, of course. The other details seem to be more important - I have my A-6 and T-500 to compare directly, but data available for the A-500 is a bit spotty overall, especially concerning pictures - this is partly due to the fact that a lot of people mistake their 500 horns for 800 horns, which leads to misdocumentation (I've been mislead by that myself). The more sophisticated 880 series altos had the newer bell-to-body brace (a two-point ring brace) for certain, as had the late 500 horns, both alto and tenor, but one can't be certain about the early 500 series.
As to the qualities of the horns, I'd like to elaborate a little: My experience - which is limited to about half a dozen horns hands-on from both the 6 and 500 series - would suggest that the 6 series are the more original and resourceful horns, but the 500 series' aren't bad by any means. Just a little less exciting tone-wise, but at least equally well built. The 6 and 500 tenors I played side by side were pretty similar, though, with only a slight edge for the 6 that didn't justify the swap. On the other hand, I compared both a A-901 and A-992 to my A-6 and I was astonished by the huge sound I got out of the A-6 - in fact, the 992 appears to be quite a different horn, i.e. it has distinctly different merits, while the 901 is more of a direct descendant. Both new horns appear to be beefier and heavier, the keywork is very slick and well thought through (though the A-6's is by no means bad and very positive, if a little less sturdy and well layed out). But they also sounded a little less lively, though more refined, especially the 992, which clearly offers a rounder and lusher core sound; it was also more neutral and less powerful when pushed. The 901 was fun to play, very responsive and noticeably brighter than the 992, but again, it didn't match the A-6's sheer power and tonal flexibility. Bottom line: The 901 is surely one of the most price-worthy new altos out there, while the 992 is certainly one of the best and most versatile ones. But the A-6 has more character and certainly power than both of them.
At $700, a well kept A-6 is a no-brainer - except if you want to use it for classical playing; this'd probably mean you'd have to look for a suitable mouthpiece to tame it (which, in a way, would be a pity).