Just to summarize the results.
Here is the breakout of the heater showing the flow sensor:
https://www.gotankless.com/stiebel-tankless-internal-view.html
This is the replacement item plus shipping:
https://www.supplyhouse.com/Stiebel-Eltron-286461-Flow-Sensor
This is what I replaced the OEM part with:
**broken link removed**
Works fine...note that on the OEM part, black is 5V +ve, red is GND...Wire accordingly and all is well.
That's $25 OEM vs $4 substitute.....and the replacement part is user serviceable!