Now We can confirm that the LM-X420 is indeed LG K12 PLUS, Thanks to a listing on Google Enterprise website.
According to the listing, the LG K12+ LM-X420 features a 5.7-inch display, 3GB of RAM and 32GB of Internal Storage, and it will ship with Android Oreo 8.1 OS out of the BOX.
We also found the LM-X420 model number in the list of Google Play Supported devices.
AS you can see from the above screenshot, The device code name is mmh4, LM-X420 is its model number and LG K12+ is the marketing name.
Our tipster also mentioned the following five model variants of the LG K12+.
LMX420N – South Korea
LMX420EM – United Kingdom -EE
LMX420BMW – Brazil
LMX420EM – European Countries
Maybe the LMX420N phone will be launched in South Korea as LGK12+, and in other markets, it will be sold as LG K40.