Damn you! You're right again - just not quite in alignment....I may be wrong about what the "80" means in terms of gvwr, it may be 18k, but I'm 100% confident that it's gvwr and not torque capacity, and that the other digits are as I described.
Wonder if @MrPink can help?
From here >> https://en.wikipedia.org/wiki/GM_4T80_transmission
This:
''The "MH1" 4T80-E is able to handle vehicles up to 8000 lb (3628.74 kg) GVWR.''
Your designation is as per the transverse installation of the '80'.