Yes, the second tag is the mystery.
The info about registering it as a lighter truck is interesting - the consignor did tell me that when he got this replacement plate he question the 4600 lbs - that leads me to believe that the first ‘new’ plate indicated the appropriate GVW.
I have found some info on Montana dmv pages but haven’t sorted through it closely enough to know if it applies in this case.
Anybody from Montana out there?