It was in 1662 that special Maundy money was minted and that practice is still continued today. Today's recipients of Royal Maundy, as many elderly men and women as there are years in the sovereign's age, are chosen because of the Christian service they have given to the Church and community. At the ceremony which takes place annually on Maundy Thursday (this year at Derby cathedral), the sovereign hands to each recipient two small leather string purses. One, a red purse, contains - in ordinary coinage - money in lieu of food and clothing; the other, a white purse, contains silver Maundy coins consisting of the same number of pence as the years of the sovereign's age.
Maundy money has remained in much the same form since 1670, and the coins used for the Maundy ceremony have traditionally been struck in sterling silver save for the brief interruptions of Henry's Vlll's debasement of the coinage and the general change to 50% silver coins in 1920.
The sterling silver standard (92.5%) was resumed following the Coinage Act of 1946 and in 1971, when decimalisation took place, the face values of the coins were increased from old to new pence.
I copied most of the last three paras from the Royal Mint web site.