St Pancras Chambers back stair 2

At first glance this staircase looks just like the other back stair, but there are a couple of interesting differences. This one has got rebates between the treads and each tread has a scalloped detail as it meets the wall. It seems odd that one has rebates and the other doesn’t – perhaps they where built by different masons? This staircase also has a tread that “flies” across the window just before the quarter landing.