Well here, at long last, they are, just three of them: the Roma, the Tropical and the simply awful Lettera 82 (also, shamefully, marketed as a Hermes Baby; Giuseppe Prezioso would be rolling in his grave, poor man).
Plastic DOES matter. These three typewriters are, as one can clearly see, essentially the same machines, mechanically - except two come in metal and one in plastic. The one in plastic is a woeful typer, the other two type very well.
The interesting thing about the Roma and the Tropical is that the mask design, by Alfons Boothby and Georges Joseph, was assigned to Olympia in 1969 and used by Olympia on the Traveller Deluxe and Japanese-made Olympiette series. There is a touch of Japanese about these Brazilian Olivettis, too, with the margin setters clearly from Silver-Seiko. Yet the Chevron, also with the same mask design, was made by Nakajima. Go work that one out.
The 1969 Boothby-Joseph design for Olympia
The Yugoslavian version of the Olympia Traveller Deluxe
The Brazilian Hermes Baby (aka Olivetti Lettera 82)