Just to add a little to the discussion, before the onslaught of Taiwanese XT clones, there were a very few vendors of 5-slot upgrade motherboards. You can occasionally run into 8 slot clone motherboards with extra pads for the 5-slot variation.

Given the limitation of expandability of the 5150, I can see why the clone makers zeroed in on 5160 layout. I think that the last board in my IBM 5160 case was a 486 board, complete with a couple of early PCI slots.

Now the trend has changed again--it's not unusual to see a modern motherboard with provision for a PCIe and a single PCI expansion.