This is a knob for older (turn of the century) Thermadore gas ranges. It's designed to fit on a 6mm "D" shaft, and it has the proper stop at the top of the shaft to allow pushing the knob in before turning it.
Two versions are provided; a two piece version printable on FDM printers (no supports needed, assemble with small wood screws), and a single-piece version printable at Shapeways.
I fabricated this using Shapeways, in their "Strong and Flexible Plastic". For more details, see the blog post on this item.