1887 Halfcrown

"Care must be taken in distinguishing the 1887 proof crown [and halfcrown] from the many strikings of the ordinary coin. On the proof, the "knife edge" of the milling is of the same width straight across, and is so sharp that it almost cuts the fingers; on an ordinary coin the indentations forming the milling have a slightly convex appearance."
Above is an excerpt from a Coincraft's Standard Catalogue of English and UK coins.
It also says the 1887 proof halfcrowns were issued only in boxed proof sets.
Hope this helps.

I'm not really familiar with the 1800's techniques of making a proof coin, but as far as I know they polish the planchet before striking. The coin on your picture looks like there's a chance someone has polished the obverse later (hard to tell from the picture though). How crisp are the outlines?