Long story short, try 433.55 MHz

I tried for years to received my Chacon Ecowatt 800 (a rebranded Efergy 2 classic) data. I knew the frequency wasn't 433.92 MHz.
I lastly opened transmitter case to find which RF chip was used ... but I didn't get any datasheet for the "A71C03AUF" I found :(

On last resort, I sought a signal on all "433.92MHz" ISM band (433.05~434.79 MHz, with sdr-sharp). I did find something centered on 433.51 (433.47 and 433.56, FSK?) MHz.
As for Somfy stuff (which uses 433.42 MHz), many generic receiver can't work with those *special" 433MHz frequencies.

In the end, I tested this new frequency using rtl-433 and guess what, there's a working Efergy module ! Moreover, I did not make a great discover, 433.55MHz is well precised in the "efergy_e2_classic.c" module comments...

RFM69 or NRF24L01 may be frequency ajustable!