When built in 1957, the whole series was U9C, 900HP. After FEPASA taking over, these two locomotives were re-engined at Casagrande Motori shops, in Chile. The new engine has 1800HP, for that reason the locomotives were renumbered from original series Dt9000 to this D1820, reflecting the current power. Unfortunately there's not much info for these locomotives available in the internet.
https://www.thedieselshop.us/Chile-FEPASA.HTML
Can you suggest a class name for these converted locos? U18C looks strange because they have different bodies.
I see your point on the denomination. I was not aware of the CM1800C denomination, it may be the correct one.