That's an amazing find. Congrats and enjoy it. The story is, engines were taken out of line for testing or repair and returned to the assembly line later. There were occasional earlier or later number spreads. The frame to engine number spread around the 10055xx range is around 400. Seen on multiple bikes in that range.
Yep, they did that a LOT before the K4 series got it nailed down better. My own K2 (11/71 build) came with a very early K1 engine in it. I think that had a lot to do with my placing well in the old "Factory Racing" days with it. The cam was an early K1 (higher lift than K2, finally wore it down by 138k miles, though), the carbs are 657a model, spark advancer the early AD125 version, stuff like that. Inisde, it had the wire retainers on the 2 innermost steel clutch plates (until I removed them), which are rarely found at all in the 750. I sometimes wonder if it was a 'junkbox engine'?

And yet, they still always run well!