I bought two different transducers: an el-cheapo langevian and a proper Branson converter. Both are 40kHz units.

Not much to see here.  The piezo is 4.4nF. Here is a first draft of the schematic of the single layer driver board, with as many components marked as I could get with a bit of googling.

On to the real prize: Made in 'Murica! Branson 4TP RF converter. my google fu has largely failed to turn up a datasheet for the specific model number, but a 4TP unit is listed as compatible with the Branson 2000BDC 40:.8 power supply, which is a 450W continuous, 800W peak output unit. The input connector shows 5.7 nF, which leads me to believe there are be 3 or more piezo elements inside.

She's a beauty to be sure. The green booster is 1:1.5, and the output horn appears to be a high amplitude gain step horn. Of course, the used equipment purchasing experience wouldn't be complete without weird incompatibilities, so while it looks like it takes BNC, the connector is actually a reverse polarity SHV connector.

the output horn is threaded with 1/4" fine (28 tpi) threads, and the hole is about 5/8" deep, so it takes hardware store bolts as viable working tips. I'll be using these once I get a tuning rig in place. I'm hoping that a longer bolt will work as a 1/2 wavelength element and can be used as the working tip for sonifying liquids.