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All of the 2XJ's that I have seen (or owned), with original bells, have had 24" recording bells and 24" upright bells. The 2 piece King 2341s had 19" or 22" upright bells, depending on the age, and 22" recording bells. I believe that the King 1241's came with 22" bells only. The only original detachable Martin upright bell that I have seen had a 24" diameter while I have seen recording bells for the same horns of 24, 26, and 28 inches.

The total length from the collar to the bell rim will have a slight impact on intonation, but the bell diameter will have less impact. In other words, if you put a 20" upright bell on an instrument that came with a 24" recording bell, it should have little impact on intonation as long as the total resulting length of the horn doesn't change.

Lew wrote: In other words, if you put a 20" upright bell on an instrument that came with a 24" recording bell, it should have little impact on intonation as long as the total resulting length of the horn doesn't change.

How would you measure the total length of the recording bell?

I'm guessing centerline length, but how would you actually measure it?