Just inserting a tiny strip of plastic or cartoon of the right thickness in the groove it works centering the 57 film in the 13x18cm holder, and then the sheet is well centered and secured.
Also you can find abundant literature about that in this forum. My CAMBO SC 5x7came with a dozen of 13x18cm holders, so I "converted" 4 holders for the moment, the posted images are in original condition, to show the little margin it remains in one side when the film is moved to the other side.
Also I sprayed one of the holders with 3M Re Mount glue, it also works OK, this is another trick, this also ensures flatness, like if it was a Sinar adhesive holder.
This is my modest DIY amateur real word experience about that.
Cheers
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