Well then, lets just say you indicate they are different. And if not identical, there must be some difference. Is it to the better or worse... You're call...
How would I logically make a call when the whole question is based on what the "U" actually means? Could be better, could be worse, could be nothing more than a "U" at the end of the model number.
I agree, but the govermential electrical testing and regulatory is for safty and not quality in general. Quality of consumer audio gear is more regulated by the DIN 45500 standard (known in the public as Hi-Fi) or AES/EBU (pro gear) standards.
Marketing is properly the one that differs the most.
Yes, I agree that UL, CSA, CE, etc. is generally for safety. But, I think it is likely mostly this aspect, regulatory and engineering, as the key drivers for different models when it comes to the service manual designations - A, U, C, E, R, etc.
Marketing can develop different literature for different regions without having a different (for lack of better terms) engineering/service model. For example, there is no M-80u sales model, just M-80. From a functional standpoint there are differences between the engineering models (A, U, C, E, R, etc) to facilitate the regional requirements of power cord styles, operating voltages, power consumption labling differences, manufacturing/bill of materials, parts catalog/list, etc.
From a sales/marketing standpoint, regardless of having different engineering model numbers, M-80 is essentially functionally equivalent around the world. One sales/marketing difference that comes to mind is the output power rating methodology - what the brand is selling and what the customer is buying. For the US, the rating typically would be per FTC guidelines, for Europe it may be per DIN or IEC power ratings. From an engineering standpoint, they're just different ways of stating capability of (essentially) the same amplifier.