Although actual greenbacks were not circulating, many local merchants accepted MPC on par with US dollars, since they could use them on the black market. This was especially evident during the Vietnam War when the MPC program was at its zenith. To prevent MPC from being used as a primary currency in the host country and destroying the local currency value and economy, MPC banknote styles were frequently changed to deter black marketers and reduce hoarding. A "conversion day" or "C-day" was the soldiers' only chance to trade in their old MPC for the new issue, after which the old MPC became worthless. C-days in Vietnam were always classified, never pre-announced. On a C-day, soldiers would be restricted to base, preventing them from helping Vietnamese civilians—especially local bars, brothels, bar girls and other black market people—from converting old MPC to the newer version. Since Vietnamese were not allowed to convert the currency, they frequently lost savings by holding old MPC that lost all value after the C-day was completed. People angry over their MPC loss would sometimes attack the nearest U.S. base the next night in retaliation. To illustrate the Vietnam War MPC cycle, in mid-1970, a soldier could have a friend in the United States mail him a standard $100 bill, take it off-base and convert it to $180 in MPC, then change the MPC to South Vietnamese piastres at more than the legal rate. The soldier could then spend freely, paying in low-cost local currency, and end the day with a sizable profit. If the $100 bill found its way into the hands of a high-level Vietnamese government official, it could be taken out of the country and deposited in a bank account abroad, where U.S. currency could be more safely hoarded, and/or transferred into a North Vietnamese European exchange account. Thirteen series of MPC were issued between 1946 and 1973, with varied designs often compared to Monopoly money due to their colors. After the official end of U.S. participation in the Vietnam War in early 1973, the only place where MPC remained in use was South Korea. In autumn of 1973, a surprise C-day was held there, retiring MPC and substituting greenbacks. MPC was never again issued, and the concept lay dormant until the late 1990s, when it was revived somewhat in the form of the Eagle Cash stored value card system, used by U.S. armed forces in Bosnia, Kosovo, Djibouti, Iraq, and Afghanistan, as well as other non-combat zones on a limited basis.