The word "siebenundzwanzig" literally means seven and twenty, i.e. 27.

"I have been faithful to you for twenty-seven years."

The phrase "ich bin dir siebenundzwanzig jahre treu gewesen" can be literally translated as "I have been faithful to thee for seven years."  Martin comes across the phrase when he is searching for what he thinks to be the instrument by which Oskar has committed suicide.  Instead, Martin comes across the airmail letter which contains the phrase in it.  It is significant that Martin is unable to grasp the implications of the sentence.  He is not able to comprehend the full range of the emotional pain that his student is experiencing. Martin is unable to embrace the Brudermensch that has become the center of Oskar's being.  This same Brudermensch that the Nazis ripped out of Germany and what will turn out to be the same pattern that Oskar replicates in leaving his wife who he thought was a "Jew hater" is the very same experience that Martin has failed to acknowledge.  This experience of humanity, something that Oskar says, "does not grow long on this German earth," is the same element that Martin cannot fully grasp.  

The emotional myopia that is a part of Martin's being is seen when he cannot probe the meaning of the phrase.  It is for this reason that Malamud does not include a translation of it in the text.  If Martin understood it, was able to comprehend it, perhaps he might have grasped the same Brudermensch that Oskar so loved and for which he yearned.  While Martin helps Oskar write his speech, it becomes clear that he fails to understand much of its meaning. Had Martin understood the meaning of the phrase in both its emotional and linguistic sense, he might have been able to understand its power regarding faithfulness and loyalty being intrinsic to Brudermensch, but lacking in a world of political and personal cruelty.