1926 King Liberty Model mouthpiece recommendations

[itsDewey]

Hello trumpeters, its Dewey. I recently bought a 1926 King Liberty Model trumpet on eBay, and she is a beaut. The only problem is that, when I try to use my modern Bach 3c mouthpiece, it slips out instantly. The trumpet came with the original adaptors, but when I play with them I'm almost a half step flat, and that isn't a good thing when in a Jazz band. It came with a bugle mouthpiece that fit and sounds very nice, but has raw brass all over the cup and I would not like to die of brass poisoning anytime soon . Does anyone have any suggestions on mouthpieces that would sound great and fit my Liberty Model?

[rmch]

I have one from around that time and modern mouthpieces work well with it, albeit they stick out just a little more than on a modern horn. The original mouthpiece still fits, too, so I'm wondering if the receiver on yours has been a little warped. Do you have a good repairman in your area? I'm curious to see what other Liberty owners' experiences are with this.
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1919 King Liberty
1923 Conn 22B
1955 Selmer 24A (Balanced Action)
1981 LA Benge ML #3 bell
1938 Couesnon Flugel
1907 HN White Co. "King" Cornet
1977 Selmer Picc