Often nation's whose economies are in the process of imploding (due to rampant mis & poor micromanagement) try many schemes to fight hyperinflation. One strategy happens to be trapping foreign currencies (or avoid exchanging their worthless money for more valuable currency) so they create a dubious foreign exchange currency to trap visitors into loosing whatever decent currency they brought in. Visitors fly into the countries are required to buy __X__ amount of these worthless coupon-like cash at the airport (& then spend it or be trapped with it).
Myanmar, Cuba & several other shady nations with incompetent/corrupt regimes practice these schemes with varying degrees of success.* So, I suspect that's the case with the 50 Won (& stamps) from North Korea. Take a look at
this link from the Bank Note Museum which explains P-21b - e. If you scroll down you will note that they produced separate "Capitalist Visitors" & "Socialist Visitors" issues in 1988 (old habits die hard). So P-21a & North Korea's economy likely started to tank, then they roll out P-21b, c, d, etc (adding stamps to notify public that they should only be used by foreigners).
*Of course what actually happens is a huge black-market emerges, using USD or other currencies of value, since people have 0 confidence in propped-up, worthless currencies.
