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The satin silver Conn 2XJ's were silver plated, and if the inside of the bell is gold colored, it is most likely gold "wash," which is a light gold plate.

Value depends on where it was made. If it is a Texas made horn it is worth less than an Elkhart made horn. I would expect it to sell for between $2500 and $3500 in the condition you describe.

I've never played one myself, but from what I've heard you need to be very wary of intonation. If I remember correctly, the F just beneath the staff is notoriously bad on the large Conns. I also believe I've heard that the bellfront versions tune better than the bell-up versions. But remember, I'm going on hearsay and a very poor memory, so play it and make your own judgment.
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Joe Baker, who could find a way to tune that F for a 24 J in really good shape.