New 2.5" SSD won't fit - my MacBook pro says it's a mid 2012 15" model So I just paid £200 for a new SSD hard drive (Crucial said it was a suitable upgrade for my MacBook Pro 15" mid 2012).

But this isn't what I was expecting to see. :(

What am I looking at? Why didn't it look like the iFixit guide?
That definitely looks like a Mid-2012 MacBook Pro Retina.  

From iFixit.com: