The RoMo cards are an interesting little side-alley in the world of Tintin collectibles.
The company - actually titled La Stereochromie - wrote the name of their brand that way, with the capital "M" in the middle, after the names of its founder,
Robert
Mouzillat, who together with his father Gustave had designed thier own 3D stereo photo camera in the early 1950s.
In many ways similar to the more familiar Sawyer's View-Master system, the most obvious difference is that the RoMo cards are rectangular, rather than circular, with the image pairs (one for each eye) side by side.
RoMo established the business with cards depicting famous places, I think as souvenirs (I recall my grandmother having similar sets of pictures of the shrine at Lourdes, and Northern Italy, which may have been RoMo cards, but I can't be certain).
Then in 1957 they branched out by contacting Casterman with an interest to doing releases based on
The Adventures of Tintin.
I've also read that Hergé may have seen a demonstration of the RoMo process at the Brussels World's Fair in 1958, leading to the approach to Casterman, but that didn't open until October 1958, so it seems unlikely, given the timescale. More likely, the company was eying up the Fair as a good opportunity to sell novelties to the visitors from all over the world, and set out to get the rights before it began.
Although the contract was agreed pretty quickly, and the choice of which adventures to do was to be made by Hergé and his studios, the artwork was to be done by RoMo, which might now seem an odd decision. Their artists used Hergé's art as the basis of their frames, but it was redrawn in stereo pairs and coloured as gouache paintings, to enhance (as they saw it) the 3D qualities.
The first part was pretty much inevitable, as the two images must be different to make the depth effect work, but the colour stage introduced shading and highlights not found in Hergé's "flat" colour, and Hergé was not happy with the results.
(To be fair, RoMo were avowedly a 3D exponent, so the fact that they wanted to bring depth to otherwise 2D picture, so perhaps Hergé should have been prepared for the additional shading...?)
In the end, only four sets of slides were produced over two years, although more had been intended, and then association was brought to an end.
The stories covered were
Tintin in the Congo,
Tintin in America,
Cigars of the Pharaoh and
The Blue Lotus, with two cards per story.
Interestingly, there is
an active website for the RoMo archives; Robert Mouzillat's daughter Elizabeth still has the original production work for the
Tintin series of cards in the collection, so presumably they could be revived if it was wanted.
I've never specifically sought them out, but I haven't seen that many sets for sale, so yes, they are most likely collectable, and they do seem to be quite rare, if only because they were only ever available for a very limited time, and didn't sell that well as far as can be told.
Do you have a set yourself?