As near as I can tell, the horn is the same one used on the original Gilbert AF diesels and the EP-5. I just compared my Gilbert AF horns with the ones on my Lionel diesels and EP-5s and they look identical. (someone correct me if I'm mistaken). Doug Peck at Port Lines Hobby Supplies has these horns for $1.00.
If in doubt, you can email Doug at doug@portlines.com and he'll be able to tell you for sure if the horns are a match.