Well that's the thing, though. There are different builds with different motherboards.
In my work fleet we have hundreds of R950s, in 3 different builds.
One build has just the integrated Intel GPU, one build has the Radeon 7570M on-board, and the build I have came with the PCIe port on the mainboard (I'm assuming it's MXM but I don't really know). These were ordered with Radeon 7570Ms, which are the same chip but soldered onto an addon card, which plugs into the port on the mainboard. The R950s with the same build as me are easy to spot because they have a much thicker 'chassis.'
Among my friends at work I'm the only one who hasn't upgraded my GPU, but we're all doing it "unofficially" with personally purchased cards, which is why I've been reluctant to open mine without a video tutorial or at least some very detailed instructions.