Maaser Sheni, Chapter Four, Mishnah Eleven
1) One who finds a vessel on which was written a kof, it is korban.
2) If a mem, it is maaser.
3) If a dalet, it is demai.
4) If a tet, it is tevel (untithed produced).
5) If a tav, it is terumah.
6) For in the time of danger people would write a tav for terumah.
7) Rabbi Yose says: they may all stand for the names of people.
8) Rabbi Yose said: even if he finds a jar which was full of produce and on it was written terumah it may yet be considered hullin, because I can say that last year it was full of produce of terumah and was afterwards emptied.
Sections one-six: According to the first opinion in the mishnah, people would write on vessels the first letter of the type of produce contained in the vessel. The reason that they wouldnt write the whole word is that in the time of danger if people were caught observing the commandments, the Roman authorities would punish them. To avoid this danger, they would write only the first letter. Hence, if one finds a vessel with one of these letters on it, he must be concerned lest the contents are that type of produce.
Section seven: Rabbi Yose is lenient and says that these letters may stand for the names of the people who own them. Therefore, we need not be concerned lest they are these types of produce.
Section eight: Finally, Rabbi Yose states an even greater leniency. Even if the jar says on it terumah we need not be concerned lest it is actually terumah, because it might be that the jar held terumah last year. Since jars are reused, we cant be certain that the contents match what is written on the jar.