Hello Sandor,
I never heard of an editor called Keller but there was a swiss collector by that name in the frist half of the 20th century, maybe he edited some figures.
By asking Google I came across this picture of a knight offered on Ebay some days ago. It was mentioned as C. F. Keller figure.
I´m pretty sure that this figure can be found in the Neckel cataloge. So maybe Neckel bought the moulds from Mr. Keller at some point.