"A grumpy old timer" indeed!
It's good to be familiar with serial numbers for a lot of reasons other than just desiring "numbers matching".
It can help when identifying the parts that are on your bike and what parts will or won't work with your bike.
It can also be a clue as to what kind of life the bike had and how well it was maintained.
For example, the 550 I'm currently working on is a '74 K0 with a '77 engine.
Because I was familiar with the numbers I was able to tell the PO the engine wasn't original and used that knowledge to
knock a few bucks off the price!
BUT this also means that I don't how many miles are on the engine. (yes, I know speedos can also be changed.)
Does it matter that the bike isn't totally original? In this case, to me, no, because I'm not planning on keeping this bike.
It really depends on what you're looking for in a bike.
If you just want a bike to chop up and customize then numbers mean nothing.
If you're looking for a good example to restore with an eye on future value then originality matters. (imho)
and find a tank/side covers that are original to the bike because you can decode the vin for the color too which is neat.
No, you can't. The VIN's on these bikes do not contain color information. Unless you can find some evidence on the tank there's no way to know what color it was originally.