In Appendix 2 of The Best Twin, Clew lists prefix 35/B twice: the first time as "350cc stationary governing engine (Marconi) sv plate head", then under the 1932 Models "Model D/E 350cc sv Marconi stationary engine, Aluminium plate heads". Yours seems to have the aluminium heads, so given the July 1931 magneto, I guess the date is late 1931 - 1932.
Sorry, no idea of value.
Cheers
Leon