Josh O.,
I'm no expert, but I'd agree: Usually, black paint indicates a Replacement/Service Use engine.
Yes, the Spec # tells us what was the intended use of the engine when it was built.
Regarding your 123 engine (model# K301AS; Spec# 4759B; Serial# C044546) the Spec# indicates it was intended for TRACTOR USE.
Although Your engine Serial# indicates a build date of 1967, which is the same build year as your 123 Cub, Aug, 1967 (Serial# 210295), that doesn't GUARANTEE it was the original engine to YOUR tractor. Your Cub may still have undergone an engine transplant early in its life.
Consider this senario: The original buyer of (now yours) 123 Cub brought his proud purchase home and promptly proceeded to cut the grass. Unfortunately, within the first hour of mowing, the oil pan drain plug became loose and it ran out all the oil and seized the engine. The sad owner promptly returned the wounded Cub to the dealer undercoated with oil-soaked grass chippings. The dealer promptly pulled out the seized engine and pulled an engine from out of another 123 the dealer had in the showroom. They gave the 2nd engine a quick squirt of black paint and then installed it into the comatose Cub to spring back to life & finish cutting the half-mowed yard the next day.
That is one example how you could have a 'replacement' engine but with a Spec # indicating FOR TRACTOR USE. The two engines would have Serial #'s fairly close, since they were at the same dealer at the same time and, thus, likely produced fairly close to each other. Does that make sense?
I wouldn't be surprised if the fuel tank has been repainted by a previous owner, since the paint doesn't stay well on the tank over the likely countless fuel spills onto it. It sounds like someone has repainted (at least) the fuel tank where the HP stick is located. If/when you remove the engine shrouding or leaf screen, look for signs of yellow paint where the shrouding sections overlap. Your engine may have been repainted sometime ago or just the tank was repainted. I wouldn't think that a dealer would paint over the HP sticker, but who knows?
If only these ol' Cubs could talk,ay? Then we could simply ask them, "What's up with that, Ol' Boy?"
Ryan Wilke