Micro HKPilot Mega Mission Planner Version

I am trying to revive an old Micro HKPilot Mega (APM 2.7.2) board and know that its only compatible with version 3.2.1 of the firmware due to size limitations.

I was using the latest version of Mission Planner and was having errors, I saw other forum posts mentioning that you needed to use prior versions of MP to be fully compatible with 2.7 boards. Question is, what version? I went back to a 2017 version 1.3.38 and in this version I get an error during compass calibration. I am using an external compass, with the trace for the internal compass severed, if that is relevant.