The 490 could be a Vega chip with not everything enabled. It would then probably have GDDR5x.
The new fury card, would be full Vega with HBM 2.0.
AMD seems to be stepping away from their earlier schemes. As in, full chip is a 390x, with the 390 being the same chip but with less clusters.
They seem to be doing something more along the lines like Nvidia. The 480 with full chip (like the 1080) and the 470 with clusters disabled (like the 1070).