TromboneChat

Hi all, I'm hoping to get some info on this trombone I found. It's an early 50s King 2B and it has these small caps on it, one on the bell and one on the tuning slide. The come on and off, and when they're on the make a flesh surface with the inside of the horn. I have looked everywhere for some version of this but can't seem to find any details...
My guess is it's some kind of modification for an attachable mic? But then why would there be one on the tuning slide? I don't have a lot of information about its history either, other than I know it came from Las Vegas. There was a trumpet with it that had the same kind of modification. Any info would be appreciated.

Thanks everyone!

A mount for a clip-on pickup. Barcus-Berry or otherwise, probably from the early '70's

Removable cap for a microphone that plugs in the horn? I think that is what Don Ellis used on his "Live At Fillmore East" album for his electronic trumpet solos on "Hey Jude". A trumpet player in my high school drilled a hole in his mouthpiece for one back in the 70s. Only reference I can find is an old thread on TubeNet:

"I am experimenting with a Barcus-Berry transducer. These types of pickups are inserted into a 1/4" diameter drilled through the exposed part of the mouthpiece shank. How does it work? Don't know yet. I have a preamp coming and will know more in about a week."

I never heard of one that fit into a hole in the bell or tuning slide, but stranger technologies have been tried...

I looked at some horns that were owned by Dick Cary. One of them had that same kind of setup in the bell. It was a hole that went all the way through the bell.