Speculating somewhat
Bill Houghton, foggy mornings in Sebastopol, CA
>I think this is right, but I'm no expert.
In classic hand tool woodworking, one edge and one face of the stock was planed true, and the two surfaces square to each other. These two surfaces were then used as the gauge against which all other work was done. A sash fillester would have been used, as Jim Reed says, to plane the rabbet for the glass - meaning that the inside of the sash would have been the "true" face against which the work was done.
That's a pretty tool, and I think you may be able to use it as a general purpose rabbet plane, which is a very useful tool. Am I correct from your e-mail tagline that you're in the Netherlands? If so, you may be not too far, by American distance standards, from at least one other member of this forum, Marc, who lives in Luxembourg.