Ma'aser Sheni, Chapter 4, Mishnah 11. In the previous Mishnah we dealt with a vessel on which the word "korban" was found written, where the ruling depends on whether the vessel is made of earthenware or of metal. Our Mishnah deals with a case in which the complete word was not written, but rather a single letter on each vessel, and the question is how to interpret what that letter stands for.
The letters and their meanings:
Kuf - represents the word "korban" (and apparently the letter was written with an apostrophe, though this is not necessarily so). It is therefore consecrated, and it is subject to the laws of the previous Mishnah, both with regard to the vessel itself and with regard to what is inside it.
Mem - indicates "ma'aser", meaning ma'aser sheni: the contents of the vessel are ma'aser sheni.
Dalet - "demai": produce that grew and was brought from elsewhere, and there is a doubt whether tithes were separated from it.
Tet - "tevel", from which terumas ma'aser must still be separated.
Tav - indicates "Terumah".
Why does the letter mem indicate ma'aser sheni?
Some of the commentators asked: how do we know that the intention is not ma'aser rishon? The answer is that it is more reasonable that ma'aser sheni is meant, since ma'aser rishon is ordinary, unconsecrated produce. And even if we were to suspect that it is ma'aser rishon from which terumas ma'aser has not yet been separated, they would not have written on it simply "ma'aser", because one could then have understood that this is properly tithed ma'aser from which terumas ma'aser has already been separated, and that would create a stumbling block. In such a case they would have written tav mem (terumas ma'aser), or mem tet (ma'aser tevel), or the like. Since a mem alone is written, this indicates that it is ma'aser sheni.
The Tosefta, however, teaches that when "ma'aser" is written without qualification, one must be stringent in both directions: perhaps it is ma'aser rishon, in which case terumas ma'aser must be separated from it; and perhaps it is ma'aser sheni, in which case it must be redeemed. In practice one must be stringent in both respects, and this is the intent of our Mishnah.
"At a time of danger":
All of this is stated because the Mishnah is speaking about a time of danger, when the government decreed religious persecution forbidding the Jewish people to keep the mitzvos. For this reason they had to conceal the mitzvos they were observing, and they did not write the word "Terumah" in full but only the letter tav, and likewise with the other letters - all because of that concern at a time of danger.
The opinion of Rabbi Yosei:
Rabbi Yosei says that these letters constitute no proof at all: it is possible that they are merely abbreviations of people's names, and they give no indication whatsoever of the halachic status of the produce lying in the vessels.
And as will be explained later in the Mishnah, Rabbi Yosei himself holds that even if the full word "Terumah" had been written on the vessel, that would not be enough to say that its contents are Terumah. His words here are addressed to the Sages according to their own approach: even according to your view, that what is written on the vessel is an indication of what is inside it, when the complete word is not written and we have only a single letter before us, there is no clear knowledge, since it may be the abbreviation of a person's name, and therefore it constitutes no proof even according to your view.
Expressing his own opinion, Rabbi Yosei goes on to say that it means nothing at all: even if a barrel full of produce was found with the word "Terumah" explicitly written on it, it still retains the status of ordinary, unconsecrated produce. For one can say that previously - yesterday or last year - it contained Terumah produce, and afterwards it was emptied and filled with other things, and what is written on it is no proof, since the owner simply forgot to erase the word "Terumah" that had been written on it.
The Sages, however, disagree and say: even if it did contain Terumah in the past, once it no longer contains Terumah, he would certainly have erased the word "Terumah" written on it.
In summary: in this Mishnah we learned the meaning of single letters written on vessels - kuf for korban, mem for ma'aser sheni, dalet for demai, tet for tevel and tav for Terumah; we clarified why mem specifically indicates ma'aser sheni, and the view of the Tosefta that one must be stringent in both directions; we saw that this ruling was stated with regard to a time of danger, because of the decree of religious persecution; and finally, Rabbi Yosei and the Sages disagree as to whether what is written on the vessel is proof regarding its contents.