Omega Speedmaster Mark II: Service Movement Swap?

I picked this up recently for a decent price. Running good. Reputable seller but not an Omega fan or buff. He was fearful of opening the watch but all else looked very good. Case is not buffed. Dial looks good. correct bracelet for a later production. I got the watch and opened it and the serial number is way out of range. I'd say by a decade.
I'm keeping it so that isn't the issue. I really trust the guy I got it from so it must have happened ten years or more ago before he got it.
Am I wrong in saying this movement came from a Speedmaster Professional ST 145.0022 from the early 80's?
Serial #45447456

I had an interesting conversation with omega today. Sadly, no extract of the archives service still. But they said the movement doesn’t show up as a Speedy Professional and they proposed it was put it the Mark 2 upon servicing. Everything else checks the boxes as original for them
Have you ever heard of such a thing?

Thanks. I posted the question elsewhere and had multiple people reach out and say they had similar experiences. All is good

My Mark II has a 31,6xx,xxx serial number and I was told it was sold in 1972 in Germany.
Might it be a service movement that was fitted at a later date?