Was the manufacturer Jewish?

Hi,

Where food utensils are purchased from a Jewish-owned store, but one isn't certain whether they were produced by Jews, one should immerse them in a mikvah without a berachah. However, where the material [like glass] is such that immersion is only required rabbinically in the first place, and one cannot ascertain whether the manufacturer was Jewish, and there are great obstacles to immersion, then immersion is not required.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 2:40)

Have a great day,
Mordechai