Also, as far as the engine number is concerned, The code may have meant something to Rootes, but they weren’t associated specifically to each engine, in fact I’d guess that someone probably just went down the line of engines and slapped the stickers on the valve covers as they were feed to the production line.
So, since the only identifier for a particular engine was the sticker, it’s not possible to know absolutely for sure that you have the original engine. One thing you can do is check the build date on the flat by the distributor. If it’s approximately 2 months before the car’s build, then you probably have the original. If not, if it’s later or significantly earlier, then it’s probably been replaced. With what will require knowing the part number on the side of the block.