Why were old pennies called 'd'?Watch
If you do not know why d for denarius was chosen please ask your history teacher because nobody seems to know the answer to this question!
Here it is on the stamps
There were 12 of them in a soludus just as there were 12 pennies in a shilling:
"With the fall of the Roman Empire in western Europe, the use and minting of coins were disrupted. With gold in short supply, most coinage systems in medieval Europe turned to silver, and the penny was the main coin, with 12 pennies (or denarii) being worth one solidus (or shilling), while 20 solidi were equal to one libra (a pound of silver). For centuries throughout Europe, one of the biggest complaints was the lack of ‘small change’, which gave rise to low-denomination tokens being issued by private traders, although base-metal coins eventually came to be minted."