Hi, long time lurker here and owner of MTX3282!
Although this post is about MTX3282, some things should apply to the newer MTX329x.
Recently I found ASYC3/ASYC4 calibration manual.
Instrument has Boot mode and there are autocal and calprint options, among others.
Boot mode is entered by pressing and holding a hold button while powering instrument on.
Please don't perform autocal, before writing down/dumping calibration values form calprint!
To dump calibration values I used TestController application from
https://www.eevblog.com/forum/testgear/program-that-can-log-from-many-multimeters/.
I've found that calibration values for MTX3282 start from offset 0xf000.
To read VOLT_AC_20M_RANGE_0_SLOPE send command:
holdingF? 0xF000
To read VOLT_AC_20M_RANGE_0_RANGE send command:
holdingL? 0xF002
... etc ...
And these values can be modified too
Thanks to RoGeorge for the Modbus documentation.