First, let me mention that YMMV depending on what part of the world you're in. Seriously. There was the YAS-25, for instance, which was available in Asia and Europe.
Anyhow, Yamaha USA's current offerings are listed
on their website. The 23 is still listed on that page, but Yamaha's Amazon page and a couple of other random dealers show that they're no longer available new. The 26 is. However, if you compare the 23 to the 26, you'll notice that the long rods for the G#/C#/B/Bb cluster on the 26 looks a lot more "Selmer Mark VI-Like" (it's actually "more Yamaha 34/52 like"). To me that means that the 26 is more of a replacement for the 275 than the 23. I briefly checked and I also can't find any new 275s for sale.
For those of you out there without tonight's line-up card, Yamaha names their horns like, "YAS-275." The "YAS" stands for "Yamaha Alto Sax." The "2" in the numbers means "student model." "6" is for non-handmade professional horns (e.g. YAS-62) and "8" is for their custom pro horns (e.g. YAS-875). After that, the numbering scheme has faltered quite a bit since I bought my YBS-52 in the 1980s. As an example, the 34 replaced the YBS-52 in some markets, but they're almost identical horns. In other words, you could end up with something like, "A 34 isn't better than a 52, but is a 475 is better than a 52?"
(There was a 61 [e.g. YAS-61] and a 21 [e.g. YAS-21]. The former was Yamaha's first pro model and the latter was their first student model.)
There was
an "O" model student horn. This was a horn contracted out to a plant in Indonesia. The "AD" horns may have been 100% farmed out to China. I haven't seen enough of them with a "Made in" stamp to know. They were supposed to have been made to be more easy to repair.
Finally, don't confuse the AD horns with the "AL" horns. Those are the "Allegro" models and were higher-end intermediate horns sold to the education market.
Recently, Yamaha has contracted out some work on their student horns to a plant they own in Indonesia -- I've seen pics of the plant and it does say "Yamaha" on the side -- and I'm fairly positive I remember seeing at least one Yamaha stamped with "Made in China."
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EDIT: I see there are 22 hours left on the eBay ad an no bids. Hey, $601 is a pretty good price for that horn, provided the ad copy is truthful.