Nothing happening on the display

I have a 2013 13" retina & a 2015 13" Retina (A1502)

I accidentally put the display from the 2013 onto the 2015 to test the 2015.

The display although is not compatible for a 2015 did give a picture.

I then put it back onto the 2013 & the unit chimes but there is nothing on the screen & just the KB lights up.. The 2013 has a new battery inside and there are no issues with that & is charging. There is no evidence that the BL fuse has blown as nothing at all comes up. External display is OK. KB & TP all OK. LCD connection is clean. Basically it boots but no picture whatsoever.

Everything is connected correctly. I have reset the SMC & PRAM a few times to no avail.

Have I wrecked the display & the 2013 unit by accidentally putting the display onto the 2015 model? A picture of what to check for internal output would be greatly appreciated. Thanks in advance for any input.