I thought we bought one as it was sold to us as such but it definitely is not a 1/4 inch when on the Singer 15-91. How do I obtain a 1/4 inch for piecing? Or do I need to resort to tape? The foot was sold for both a low shaft Singer 66 treadle and an electric low shaft Singer15-91. They are supposed to both use the same feet. Where we told wrong by the seller?