Topic: I know I know... but seriously. What's up with this serial number?!? CB750

[Tugboat]

AHHHHHH the suspense is killing me!!!

[KingCustomCycles.com]

The plate is only pop-riveted on, I would drill them out and take a look underneath for numbers.
 It could be a replacement frame and it's blank.
 I remember changing frames and we had a lot of trouble getting 6mm stamps, no way was the boss buying a set for one frame every year or two ( they were not cheaply available in 1977/78, no Harbor Freight, 'Made in China' stuff around)
Could be they had a plate made and just riveted it on?

That's kinda where i was going with my post. 
I have a bike with a replacement frame and the new number is crudely stamped on the head, but as you suggest they may have made up the plate.
I'll bet there's nothing under that plate.

How certain are you the frame is a replacement frame?  Maybe the PO told you it was? :-)

[Tugboat]

I am not at all sure. The PO told me nothing.

[CycleRanger]

How certain are you the frame is a replacement frame?  Maybe the PO told you it was? :-)

It has a state-assigned number.  It's TX0000123 or something like that.

AHHHHHH the suspense is killing me!!!

Here's another thought, your frame is a factory replacement frame from another bike! 
That's why it's marked as a K.

[KingCustomCycles.com]

State assigned means it was titled as owner built, assembled, custom or it was reinstated after being declared a total loss by the insurance company. 

[CycleRanger]

State assigned means it was titled as owner built, assembled, custom or it was reinstated after being declared a total loss by the insurance company.

Yeah, it was backed over by a road grader back in the '70s. (At least that was the story.....)
That's why the tank, side covers, and pipes look practically new.
I got the bike for less than what the pipes are worth.
Although the oil tank still has a big dent in it.