latest volume, [Kelderek Owsla]
https://hobby.dengeki.com/aoz/illust/reboot81/

>GM Quel was the mainstay MS of the Titans when they were formed and served as the basis for the test units RX-121 TR-1 [Hazel] and RX-122 [Kelderek]. Thanks to that, they inherit some parts of the structure of the RGM-79 series, tracing their lineage all the way back to RX-78-2 Gundam. GM models maintained compatibility with RX-78 series weapons and assorted gear, and this is still retained in these new Gundams.
>Though they may seem very similar, [Kelderek] is not a variant of TR-1 [Hazel] but its own distinct unit with a different concept behind it. Where [Hazel] is intended for testing new equipment as part of the Gundam TR-S [Hazel Hrair] development process, [Kelderek]'s purpose is testing specialized equipment from the One Year War and the post-war era. The data gathered from that was used to develop an operational system for [Hazel Hrair]'s upgrade parts, later known as BUNNyS.
>However OYW era mobile suits are not compatible with Transpack system, the new backpack connection standard. In order to rectify that a new joint was developed, called a "Grapple Arm", enabling usage of previous generation equipment that was not developed with compatibility in mind.
>Though not a TR unit itself, [Kelderek] is a machine that that contributed to the TR Plan by testing gear and software, aiding in the development of the advanced equipment control systems used in TR-S and TR-6.