What exactly is a “2-3-1 Pacific”?? The proper term is a 4-6-2 Pacific. The Whyte Notation for a locomotive is the lead truck (pony truck) by counting all the wheels (both sides), the drivers (middle powered wheels) as the middle number, and the trailing truck (rear truck) as the third number.
https://en.m.wikipedia.org/wiki/Whyte_notation
There are also a lot of MTH Renditions. Photos and/or the model number will help tell what locomotive you are referring to. I can name atleast 6-7 different Pacifica that MTH has made in the past 5-6 years. Let alone the 30-40 they’ve made since they were formed.