I think you are right. When I first saw the toy in Target I wasn't even aware of the Masterpiece line as I wasn't keeping up with toy news. Although from my childhood I knew about the original 'G1 white Prime' Ultra Magnus but it didn't dawn on me when I saw the box and the first thing I thought was "Why do white Prime??". I knew it was legit because I was familiar with Takara and that they did their own releases but aside from that, I knew zero about the toy or the MP line.
Well, I bought him anyway as it was the last one, the box seemed kinda light for its size but I could tell that there was stuff inside (a lot of rattleling) but because I wasn't at all familiar with the Masterpiece line, I didn't think much of that. I bought it, took it home and then opened it....and Bang! The toy in the box wasn't Ultra Magnus but some half assembled purple model kit of some insect like alien spaceship that I could not recognize! The parts were mostly loose and what was assembled looked wrong. By now I had read 'Ultra Magnus' on the box and remembered the G1 'white Prime and this thing looked nothing like it.
I soon realized that some jerk had stuffed the box filled with junk and then returned it to Target for a refund who they simply placed back on the shelf without checking. immediately I went online and researched the Masterpiece line and became educated in exactly what the line and the toy was.
The very next day, I went to my local Target (different Target from purchase) and told them of the situation. They were kind enough to make me exchange it and thankfully they had a proper MP Magnus in stock! I felt so relieved but also a bit peed off that there wasn't a Trailer!
BTW: It cost me like $99.95