May I ask, who returned the letters? Because i asked my 2 science recommenders (last minute) to place on letterhead and sign. One of them even said, it doesn't matters, but if you think it would help, then I will. He isn't mean or anything, he just stated that he has, in the past, written letter w/o letterhead. So, hopefully my 2 science recommenders placed their letters on letterhead. Also, my dentist 100% did not put on letterhead, bc he told me same thing as OP. And my research professor, at another school would be classified as research professor/employer per some school requirements.