They are simply different conventions for describing tyres. The M series are US possibly North American spec, not commonly used elsewhere, whereas Metzeler is a European company. There are quite probably even more ways of describing the same tyre!
Metzeler 888 are listed in both 130/90-16 and MT90-16 with the MT90 having a very slightly higher Load rating (73H vs 74H)
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The H is nothing to do with the load rating.
Only the number, 73 or 74 is anything to do with the load. 73 = 805lbs |-| 74 = 827lbs
The letter is the max speed index. H = 210kph / 130mph
Should not mix rating category values when replying.