פירוש על עבודה זרה 138:7

This is R. Yohanan’s answer. R. Shimon b. Gamaliel did not know what type of stopper the rabbis were referring to. If they were referring to a clay stopper, then the stopping could be seen on the inside or outside. This is what he said in the baraita. But he also said to them that if they were referring to a lime stopper, the stopping can still be seen on the inside. This will deter the non-Jew and the wine will be permitted under the assumption that the Jew did not leave the wine alone with him for long enough.

According to R. Yohanan, the rabbis disagree only concerning the lime stopper. Since the stopping can only be seen on the inside, the Jew might not turn the stopper over to see if it had been tampered with. Knowing this, the non-Jew might bore a hole and drink some wine. Therefore, if he has long enough to do so, the wine is prohibited.