Low Bb too flat on curved soprano

I've got 2 vintage straight sopranos and 2 curved. I can get them all to play in tune except for the oldest Carl Fischer Buescher stencil from the 'teen's that doesn't even have pearl keywork. I can play it in tune (well...) except for the low C# down to Bb, where the notes get continually worse out of tune to where I can actually lip down to A on the Bb. I've tried to push the mouthpiece in, even filed out a groove to curve around the octave pip. I had this problem on the other curved, a Martin stencil Symphony, and the groove solved my
problem, but not with the Carl Fischer. Could it be the sax, or do I need to buy even more mouthpieces?! I have vintage pieces that work well with my straight ones, also a Meyer 9 and a skinny Runyon 6 that all work fine. I've tried to search for a similar thread but need help on this one.