MP17 US MPC currency Series 661 5c, 10c and 50c

Description

See Photo:
In use October 1968 to August 1969
Military Payment Certificates, also known as MPC, were a form of paper money issued by the U.S. Government to pay their personnel stationed in foreign countries.
It was an effort to prevent overseas black-marketing – by making it impossible (or very difficult) to take the profits home.
Off-post overseas merchants and individuals were forbidden to possess them (by local laws).
The notes were not legal tender in the U.S., and could not be banked or spent there.
The notes were used in various foreign areas across the globe, beginning in 1946 at the end of World War II; until the last certificates were withdrawn in 1973, just after the end of the Vietnam War.
In total, 15 different series of MPCs were designed and
produced.
However, only 13 series were issued for use to pay military personnel.
The remaining two issues that were never used were supposedly destroyed.
Also, MPC designs/series were frequently changed to deter these black markets from developing.  A conversion day or "C-day" was military personnel's only chance to trade in their old MPC for the new issue (and there were limits).  After the trading, that window closed, the old series of MPCs became worthless.
C-days were always classified and never announced in advance to the troops.  When the C-day was announced in the morning, all troops were restricted to base.  Guards were posted, and nobody could then enter or leave the base.  This prevented soldiers from leaving the base and helping local establishments trade in their MPCs for the new series.  In the early morning on C-day the troops were required to turn in their MPC cash, and they were given a receipt (and there was a limit).  In the afternoon, they turned their receipts, were issued the new series MPC cash, and the old series MPC notes became worthless (unconvertible).  All of the turned-in MPC cash was then destroyed by burning it (in Seoul, Korea, was done at the Yongsan central heating plant).  Not very many notes survived, since they were a monetary loss for any who withheld them.  Now, 49 years later, surviving notes of this unusual U.S. currency have become very collectable.
I personally went through two of these C-day conversions in Korea.