My list doesn't suggest which one is right for the SAEC 8000/ST. The SAEC 8000 has a slightly longer effective length than the SAEC 506/30, but also has a slightly longer overhang so my bet is that most appropriate is going to be an AX-6G. They are available from time to time on auctions and used sites. There is a guy here in Japan who makes them as well, selling them for JPY 28,000 (all told, about US$300); they are exact lookalikes to the originals save the "Micro" engraving.
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The guy to whom I was referring is here. Disclaimer: I have never dealt with him. His stuff can also be purchased on Yahoo! Auctions in Japan. If you need a link to his auctions there, please let me know. Enjoy. I would love to get my hands on one of those arms.