How do you contact a birthparent that doesn't know about you?

My situation is a bit unusual. I, myself, am adopted by my step-father, but I'm also a birthmother who placed a son 5 years ago. My mom also placed a baby for adoption in 1971. We were reunited with my sister in 1989 and remain very close. Here's my question now. I am currently trying to help my sister find her birthfather. Basically my mom had just graduated HS and this guy came into her small town to be a field worker for the summer. She got pregnant and moved to an unwed mother's home, and placed my sister for adoption. My mom doesn't remember the birthfather's name. She only remembers that he attended a local university as a freshman the school year before he came to her town, and that he had brown hair/eyes and was a couple of inches taller than her. Anyway, I contacted that university and was able to get a copy of the year book for that specific school year that he would have been a freshman. We have it narrowed down to about 15 guys, and my mom just can't remember who he is. I don't know if it's because she doesn't want to remember, or if she blocked it all out because of how traumatizing it was for her. So basically I need to write a letter to these 15 guys, and say hey, did you work in this town during the summer of 1970, and if you did, hey, guess what? You have a daughter that you don't know about! I have NO IDEA how to write this letter, but my sister really wants to find her birthfather, and I have no idea of how to go about doing this. I'm sure these 15 guys aren't going to like receiving a letter in the mail like that, but what else should we do? Do you have any advice on how to word this letter or another idea all together? I don't want to scare the man off. :) Thanks so much for all your help in advance!! Dani