If I had to guess, Class B is probably the intermediate version of the old Class A. My Class A plays wonderfully, and was originally in high pitch too. The previous owner had tubes soldered onto the existing tuning slide receivers that lowered the pitch to 440. I don't know if it's worth the cost, but maybe that's a decision for after you get the horn. For 50 euros (~$52), you can't go wrong. I'd jump on it, if it turns out to be junk sell it.
1905 Boosey Class A Euphonium-Wick SM4M
Yamaha 301M Marching Baritone-Schilke 52
1960 Conn 11J-Conn Helleberg
1961 Conn 14J-Vincent DFL
2015 King 2341-Bach Corp. 24AW
Olds O95 Sousaphone-King 26