Responsa for Shabbat 115:12

ה"נ מסתברא מדקתני זה וזה אין מקבלין טומאה אי אמרת בשלמא של מתכת הני הוא דלא מקבלי טומאה הא כלים דידהו מקבלי טומאה

— No. In both cases his master had set it upon him, but one refers to a metal [seal] while the other refers to a clay [seal].<span class="x" onmousemove="('comment',' It is shown below that this must refer to a metal seal; hence even if his master set it upon him he may not go out with it, for should it accidentally snap off the slave would be afraid to leave it in the street on account of its value, but would bring it home, which is forbidden. But the value of a clay seal is negligible, whilst if his master set it upon him he is certainly afraid to remove it; hence he may go out with it. Consequently, the prohibition in the Mishnah, which treats of a clay seal, must refer to one that he set upon himself. ');">¹²</span>

Teshuvot Maharam