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From your pictures the sizes seem the same though, are the 78 different from the 77? It wouldn't be unusual, I think I had (may be still do) the exact same model and number in two vastly different sizes, like 4/8 and 7/8, but I don't remember if that was dovo or puma.
Following my opinion, 78 and 77 use exactly the same blade (put one over the other and could not see any difference). Sizes vary all over the set about 1mm, and you cannot say 78 is larger then 77. The 78 are inscripted 5/8, for me its 6/8 (see the picture), but i know this from older TIs. Afaik, the 77 is build between 1930 and 1940, the 78 may be the succession model. The only difference i can see: the black in the etching seems to be more intensive.