Surface Book Performance Base Interchangeability?

[jaypro]

I'm looking at a local listing for a Surface Book, but the configuration seems odd.

 

The listing says the specs are

i5, 8gb ram, 256gb top

965m bottom

 

However, from my research it looks like this was never offered as a possible configuration.  As far as I can tell, the i5 model with dedicated gpu was only offered with the custom 940m

 

So I'm wondering, if the performance bases are interchangeable and the seller actually has mixed two surface books?  Or somehow managed to trick Windows into thinking the GPU is actually a 965M?  What does the 940m variant show up as in Windows, since they marketed it as 'Custom Nvidia GPU'.  How should I check if the actual performance base is indeed a 965m?  I believe the 965m base is thicker than the 940m, should i just take a ruler or is there an easy way to spot the physical difference?

 

[Shiv78]

Yes, they are interchangeable.

 

965M base has a visible bulge with a very noticeably larger vent: (edit, pic didn't work)