7370 blink code - 3 times white - no boot no post

Hello from Germany!

Got an 7370 Latitude here w/o battery, power supply, SSD and a missing keyboard cap.

Configuration, learned from the Service Tag/Express Service Code:
Intel i5
8 GB RAM

After inserting a 256GB NVMe (works fine in another Latitude) and using a power suppy from my Latitude 5520,
the power switch turns on white and stays on for about 10 sec. Than the LED at the front (under the touchpad) starts blinking white 3 time - short pause - blinking white 3 time - short pause - ...

What I have done so far:
remove power supply, disconnect coin cell overnight, power drain.
No boot/splash screen shows up, no post, no diagnostic, no BIOS setup...

What to do in this case?

BTW: entering Express Service Code, Service Tag or Latitude 7370 to the "Add Product" part of this threat results an error: "not found". Entering the same infos at the Support Page for driver will bring the correct machine.