There were two forms of threepence, the currency issue (with a duller surface), and a special piece (like this one) for the Maundy Thursday ceremonies - both with numeral 3 on the reverse but the Maundy issues have a proof-like finish.
Maundy coins are specially minted in varying (but always small) mintage for ceremonial purposes and not intended as circulating currency. They are handed by the British monarch in person to selected deserving individuals once every year. They are issued in "prooflike" condition and usually remain in it, especially the more recent ones.
The Royal Maundy is an ancient ceremony which has its origin in the commandment Christ gave after washing the feet of his disciples on the day before Good Friday.
The commandment (also known as a 'mandatum' from which the word Maundy is derived) ‘that ye love one another’ (John XIII 34) is still recalled regularly by Christian churches throughout the world. The ceremony of washing the feet of the poor which was accompanied by gifts of food and clothing can be traced back to the fourth century.
It seems to have been the custom as early as the thirteenth century for members of the royal family to take part in Maundy ceremonies, to distribute money and gifts, and to recall Christ's simple act of humility by washing the feet of the poor.
Henry IV began the practice of relating the number of recipients of gifts to the sovereign's age, and as it became the custom of the sovereign to perform the ceremony, the event became known as the Royal Maundy.
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 VIII’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.
Apart from the surface quality, circulating and Maundy threepence coins are indistinguishable. For the purpose of completeness though, the circulating coins are listed separately in the (normal) circulating coins section.