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I have a 1926Conn 2jEb. When I play on a denis wick 5 I am very flat. I can lip it up but intonation is squirrelly. When I play on a coast mp ( nothing on it other then coast), I can lock intonation with almost no lip adjustments. Why???
Also how flat does a horn goto if playing in a 50 degree room

Try to pull out the cat, it might help?

That's a small mouthpiece for that horn. Because you did not put an L after the 5 it is likely a small shank. Horns in the 1920's and before are likely high pitch or low pitch. That means they tune to something lower or higher than the now standard 440. If it hasn't been go through by an experience technician, it may not be able to play in tune with any modern band. Additionally, the length of the shank of the mouth piece can be different or the mouthpiece goes in farther. All these things can affect the intonation. When I change mouthpieces, I have to go through and retune each finger combination and find the best slide positions for a that mouthpiece. Lastly, some mouthpieces just don't work with a given horn. I just can't get acceptable intonation with it. That's why many of us go on a mouthpiece safari to find the one that works be for us and the horn. I have a different mouthpiece for each on my horns.