According to the developer, who has provided a number of accurate tips on HTC in the past, the 5-inch Nexus phone is codenamed the T50 and the 5.5-inch model is the T55. Obviously, the codenames are derived from the screen size of each unit.
The Nexus One, the first Nexus device manufactured, was produced by HTC back in 2010. The company is also responsible for 2014's Nexus 9 tablet. Last year, two new Nexus phones were launched. LG released the Nexus 5X and Huawei delivered the Nexus 6P.
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source: @LlabTooFeR via GadgetzArena