Phone blocked on JB: impossible to flash RUU

Hi to everybody, I've bought an used HTC One, I've do S-Off, root and everything else.
But after I discovered that it has only 16Gb and not 32Gb and it is the U version, so not LTE available.
So, acting like an idiot, I've tried to restock to send back the phone.
I don't know what kind of ancient Ruu I've flashed so hboot now is 1.44 with Android 4.1.2.

This are all the operations I've done to restock:

1. Change the CID into HTC__001
2. Locked the bootloader
3. Trying to flash PN07IMG_M7_U_L50_SENSE60_MR_HTC_Europe_7.19.401.10 2_Radio_4T.35.3218.16_10.33Q.1718.01D_release_4218 04_signed.zip, as I friend of mine has suggested
But it does not work, everytime I've tried to Flash I've get an error and the process hangs down.
I cannot find any other RUU for HTC One M7 U with KitKat, or any other to update the hboot...

What can I do now, I can only use the phone with JB?

Thank you so much! :)