I think that it could be as simple as a previous owner replacing a worn out sole on the plane.
The toe seems to wear faster as evidenced by the wear on the existing plane, and it could simply be that the screw attachment was to allow easy toe replacements when needed.
Here is an adjustable mouth rebate plane by ECE - not a canted blade though.
The whole toe can be adjusted forward and back and the adjusting screw sits in an elongated hole for the purpose
Cheers
Tom
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