I swapped my A51 R2 X99 XJKKD mthrbrd into my R1 finally ... I bought it used on Oct 9, 2017, & 2 years later to the day - after getting it the final parts it needed to work (CPU / DDR4) - it's up & running
(2year old receipt, lol)
I've got another pack of 2x4Gb DDR4 3200 coming, a WiFi...