Hi all,
Gold was scarce in that period. Supplies of gold dry up. Gold flowed abroad to the Sassanids who demanded large sums of gold. Also a lot of gold was paid to german chieftains.
So, gold drained from the roman monetary system. When I google on pictures of coins in 238 AD, it's just silver/ metal ones.
So I think your mouse mat is one for coin-collectors.
Maybe someone with such an interest could confirm this.
Auris Arrectibus