I musta missed this two weeks ago, as they have now been released.

A set of four translucent grey Microns (from previously released TFPrime toys in Japan) were announced a couple weeks ago as a purchase incentive (which is pretty common in Japan now) at select retailers from April 28th.

Shadow Ciel/C-L (from AM-03 Cliffjumper) 3000yen purchase at Toys R Us stores.
Shadow Are/R-A (from AM-04 Ratchet) Optimus AM01 purchase at (ion toy department?)
Shadow Gora(Gorilla) (from AM-05 Megatron) 3000yen purchase at Joshin, Bic Camera.
Shadow Balo(Buffalo) (from AM-06 Skywarp) 3000yen purchase at Yamada Denki, Yodobashi, Edion.

Now that they are out, there are some more pics of the four Microns here, with some potential combiner combinations.