Cartridge without serial?

@chakster  You explanation would apply for a very large conglomerate that sells very high volume.  I highly doubt that any of the cartridge manufacturer's are in that league. Surely they must know all of their distributor's and exactly what product they are shipping to them. I would be surprised if Nakatsuka-San is making and selling a hundred pieces a year-- in total! 
Therefore, it seems odd to me that this situation is going on, unless of course something else is occurring ( which I suspect strongly that it is!).

cartridge without a serial number would make me wonder if its real, knockoff/or stolen.   

@daveyf even if you know your distributors how you can catch one who sold to grey market dealers a bunch of them (if serial numbers replaced with fake number)? This is the only reason to remove or to replace serials, this is ID of the source. 

Okay nowadays it is possible to fake everything, but faking entire cartridges could be pure fantasy.
It could be stolen material and consequently the serial number label is removed if it is not engraved on the same body cartridge.

If a cartridge sold with fake serial number (or without serial number) then you can only guess if this cart passed quality control or not, the history of this cart is unknown. What is easy to do is to make it looks like sealed and never opened before in the box. On my sample the field for serial number in the manual was blank (it was a grey market sample from juki), but my next sample from Mehran (official US importer) was with serial number also printed in the manual and on cartridge body.