Avodah Zarah 39a: Milk, meat, wine and techeiles delivered by a non-Jew require two seals.
Shulchan Aruch Yoreh Deah 118:1: Wine, meat and cut fish that was entrusted or sent with a non-Jew requires two seals.
עבודה זרה לט ע”א: אמר רב: חבי״ת אסור בחותם אחד… חלב, בשר, יין, תכלת אסורין בחותם אחדֹ.
יו”ד קיח ס”א: יין ובשר וחתיכת דג שאין בו סימן שהפקיד או שלח ביד עובד כוכבים צריך שני חותמות.
In the early days of Lakewood, when Rabbi Ahron Kotler zt”l was the rosh yeshiva, there were 58 bochurim and 18 yungeleit learning there. The yeshiva used to buy eggs from the various egg farms in the area. Once, one of the farmers, a religious Jew, donated 180 chickens to the yeshiva. As these chickens could provide meals to the struggling yeshiva for a whole month, it was a well-appreciated gift. He brought them to the shochet, and then to the plucker, and then sent them to the yeshiva in a delivery truck driven by a non-Jew. There was no identifying sign on them, so the bochurim asked Reb Ahron what to do.
Reb Ahron replied that if the shochet confirmed that he indeed slaughtered 180 chickens, then these could be assumed to be the same ones. “But don’t we need two seals?” someone asked. Reb Ahron explained, “Here in America there are two kinds of chickens: meat chickens and egg-laying chickens. The egg-laying hens, once they grow old and can no longer produce enough eggs to be worth keeping, are slaughtered, but their meat is very tough and difficult to eat. It is usually used for canned soup, or made into pet food. This is the kind of chickens the farmer was sending to the yeshiva. Why do we need two seals? Because we’re afraid the non-Jew might steal the meat and replace it with non-kosher. But no one would ever steal and replace chickens like these.”
The bochurim in yeshiva had to cook those chickens for eight hours before they were edible.
Source: Rabbi Moshe Heinemann
[There are two separate decrees of Chazal: the rule of בשר שהתעלם מן העין אסור – meat that was out of sight is forbidden, because we fear someone may have switched it – and the rule of שני חותמות – that one sending meat with a delivery man must put two seals on it.
The first decree is the opinion of Rav, and the Gemara in Chullin 95a challenges Rav from several baraisos. Rav successfully defends himself, and the Mechaber in Yoreh Deah 63 paskens like Rav; however, the Rema, following the simple meaning of the baraisos, paskens not like Rav.
The second decree is universally agreed upon, and the difference is as follows. Ashkenazim who follow the Rema are not worried that someone would steal a piece of kosher meat and replace it with treif meat. If he wished to steal, he would simply steal and not replace it at all. But a delivery man has the responsibility to deliver the meat. If he got hungry on the way and ate the kosher meat, he would have to replace it, or else he would be out of a job. Therefore, we are afraid he might buy treif meat to replace it. The two seals ensure that this is the original meat sent.
In Reb Ahron’s case, the delivery man would have had no motive to steal and replace. The only problem would have been “meat that was out of sight” – but Ashkenazim hold like the Rema who permits that. If there were any Sephardi boys in Lakewood at the time, they would have had to find something else to eat.]
Read and Remember 