The B-2101 and B-2102 are similar models and are separates corresponding to the AU-G99X integrated. They were meant for the international market and, according to Morden's Sansui data base were produced from 1984 to 1986, like the G99X. These are all quality amps with Sansui sound but come from the time when build quality began to deteriorate, i.e. more plastic less metal. The AU-G99X is similar in sound and design to its predecessor, the much admired AU-D11 II.
http://akdatabase.org/Sansui/Report1.pdf
The B-2102 MOS vintage is an entirely different beast. I've never heard one. It it appears from the Best of Sansui site
http://www.sansui.us/Issues_IntvsDomestic.htm
and the Japanese audio heritage site
http://translate.googleusercontent....le.com&usg=ALkJrhjjky52jd7JnHsPdLwn6-vzJ8bdWg
that the The B-2102 MOS vintage was a high priced amp intended only for the Japanese market produced in 1988. If you look at the Japanese models listed on the best of Sansui site, you can see that in the Japanese market model numbers were frequently reused. For example, there is the D907 (which corresponds to the AU-919), the D907 LIMITED, the D907F (our AU D11), the D907F EXTRA, the D907G EXTRA, the D907X, the D907X DECADE, the alpha 907, the alpha 907i, the alpha 907i MOS LIMITED, the alpha 907 EXTRA, the alpha 907L EXTRA, the alpha 907DR, the alpha 907KX, the alpha 907XR, the alpha 907 NRA, and the alpha 907 NRA II. Most of the later models available only on the Japanese market were 100 V only models like the B-2301 discussed in another thread. So, it's no wonder you are confused. I hope this helps.
Marc
