Apologies in advance if this is widely known.  Personally, I've never found a really good explanation of why front anamorphs produce oval bokeh and rear anamorphs don't, despite reading my fair share of patents, technical papers, internet gossip and the like.  Feeling that my own understanding needed some firming up I finally set up some paraxial models and went through the math in gory detail.  It all boils down to how front and rear converters alter (or don't alter) the f/#, and basic DOF type