Here is another way to look at how the story of the widow's mite can
help decide what the
tribute penny was. The important point of the widow's mite story is that even though the widow gave a small amount of
money, she gave all she
had. The specific coin or coins was not important. Since Mark was
writing for a
Roman audience, he could have simply written that the widow contributed a qudrans. He did not have to write that she gave two lepta, which is about equal to a qudrans. It would not have made any difference regarding faith or morals, but for some reason Mark wanted to get the details correct, and so he specified the exact coins the widow
had, then explained what they were valued in
Roman money.
In the story of the
tribute penny, Mark could have written that Jesus was shown "a coin" or "a coin that is about the same value as a
denarius". Instead, he wrote that Jesus was shown a
denarius. I think it is probable that Mark wrote that it was a
denarius because, just as in the story of the widow's mite, he wanted to get the details correct. The correct detail is that the coin probably was a
denarius.