Question about Laptop motherboards

Greetings,
So I have a Toshiba Qosmio X505-Q887 laptop that needed a new motherboard. The video card was shot and my attempt at performing a reflow failed.
When I found the board on EBay I compared the model number and at first glance it looked like the one I needed.
The model number is                               A00005090
But the one I bought was actually   A00005040
I didn’t notice the last two digits were different until I received it but they look completely identical
In every way so I went ahead and installed it.
Unfortunately I still had a black screen but the VGA output was working fine with an external monitor.
I replaced the entire monitor with an identical known good one from another Toshiba X505 –Q887 but still have no video.
I have updated the bios
Updated the video driver
Installed two new memory modules but still not even the slightest bit of light emanates from the computer monitor.
My question is this,
Could using a MB with a slightly different model number have an effect on this?
I can access window through the external monitor, and it does show up in the device manager and indicates that it is working properly.
The function keys work but have no effect on this problem.
My thought is that the monitor is really supposed to be paired with the A00005090
Board but if that’s the case then why does everything else work.

thanks in advance
Larry