I found 352 STELLWAGs in the phone directory.
I have never heard of the name, it is in none of my books, so all I can do is guess.
It reminds me of STELLMACHER, a German expression for Cartwright. The STELL part is short for Gestell (frame, chassis). WAG could be short for Wagen (cart) or Wagner (again: cartwright).
Sorry, this is all I can think of. Maybe someone has a better idea ...