This is what I found: if you take an E-flat alto and play it like a tenor trombone it becomes a transposing instrument in F.Moreover, when sixth position F (F3) is taken as written middle C (C4), this transposition is the same as a French Horn. So, an E-flat alto played as a tenor can read F-Horn music.
A layman’s way to the alto trombone
New Quadrivium 