Extreme Summit X450-G2
Introduced June 2015 -- according to Network World
The X460-G2 switch, announced October 2014, is very similar to the X450-G2. They differ in stacking options.
The data sheet dated 1 year after introduction says that the whole family of Summit switches can be stacked. SummitStack-V is compatible with Summit X440, X450-G2 switches with 10Gb uplinks, X460, X460-G2 with 10Gb uplinks, X480, X670, X670V, X670-G2 and X770. There is no industry standard for stacking. A proprietary tag carrying packet metadata like port of arrival and output destination port is attached to every packet placed on the stacking bus. Any switches that are stack-compatible are drawn from related silicon.
Noting that the X770 has been ID'd as a Trident II, Extreme appears to be a Broadcom shop.