My congratulations! Even standard (not proof) coin is quite rare. I know that there were two varieties – with and without mint mark (letter B on the king's bust). It was struck at the Bombay Mint. 

Also, there were two varieties of 1903 year, produced by the London Mint: figure 3 of date almost touching the cipher 0; figure 3 of date distinctly separate from 0.

Remick had three Straits Settlements 50c Edward Vii in his collection, which was sold by Spink, Part 1, 18 October 2006:

lot 495 50 cents 1902, almost unc, sold for £380.-
lot 496 50 cents 1903, ef to about unc, sold for £600.-
lot 497 50 cents 1905, proof, B incuse, (KM23; Pr.-), brilliant, attractive colourful tone, almost FDC, rare and pleasing, this date only struck as proof, sold for £8000.-

Perhaps lot 497 was graded later and it is your piece? Unfortunately the photo in Spink's catalogue is not the best, so comparison with your photo is not really conclusive.

Thank You for offered versions regarding provenance of Straits 50C 1905B Proof. Perhaps it's really Lot 497 from Spink auction in 2006. Unfortunately I don't have printed Spink catalog. Coin is also unlisted in www.coinarchives.com. Thank You for help.

I also emailed Stack's Bowers cataloger but didn't get an answer from him.