O.K. I never heard of it either. My S.O. grew up in Indiana and worked at her father's restaurant. It was not just their difinition but quite regional.
Beef Manhattan is a Hot Roast Beef Sandwich with Mashed Potatoes and gravy. I guess the name eliminated the confusion of a roast beef sandwich heated, or a "Manhattan."
I will leave the definition of Turkey Manhattan as an exercise for the reader.
The BEST she has found "Out West" is at James Restaurant. Huge portion of REAL mashed potatoes and good roast beef over bread and all smothered with a good "sauce", what the menu calls gravy. All nicely seasoned. She took 1/2 home for another delicious meal. (And she is a BIG eater!)
Actually it is more of a coffee shop but everything we have gotten has varied from good to great.
739 Truman St, San Fernando, CA
(I don't think it's finished. The links don't work.)