Here's a fun widget that we've been working with IRD to develop.
It's a 50.4 BCD chain ring designed to mount to french style 6 bolt cranks. It has mounting points for 130, 110 and 94 BCD chain rings so you can dial in your gearing for different riding conditions.
They're specifically made to fit the Sun XCD cranks in the picture, but they should work well with other modern reproduction crank arms with the same bolt pattern.
Since they'll work with most commonly available double chain rings you'll never have to scour ebay looking for NOS TA or Stronglight rings again. The cutout design matches the chain rings that come on the IRD Defiant cranks, and it will work with the Defiant replacement rings that are now available.
They have shift ramps and pins like any modern chain ring, so they should shift with indexed derailleurs as well as classic friction systems. We should have our first production run in a few months.