Then I guess that there is 2 possible causes:
1. The mouthpiece receiver may be not of a modern standard and your modern mp may be going a little bit too far in
2. The mouthpiece throat may have been tampered and again the mouthpiece is going again too far in.
There is a 3rd not too likely cause - your cornet is high pitch - not very likely with capri cornets to my knowledge, though I am not a vintage expert...Dale? A.N.A.? Any thoughts?