The slot is factory but it's worn on the one side. The later sub frames had tabs welded on to make a wider bearing surface which helped slow down the wear. The rockshaft on the tractor that the slot slips over will also have some wear. Here's some images from two mower deck manuals that show the slot.
Hey Harley - in the first pic that Kraig posted it shows the slot all the way against the frame cross bar. Your slot is obviously worn close to the open end of the slot. Don't worry about your subframe not sliding all the way onto the cross bar. They typically don't. The airbrush artists at IH were notorious for drawings not quite correct. Just ask Kraig. Maybe he'll even start a thread just for "drawings not quite correct".
Hydro, I believe when the deck is fully raised it is nearly fully into the slot, as the deck is lowered it moves to the outer edge of the slot. As most mowing is done with the deck lowered and typically in the float setting, most of the wear occurs at the end of the slot. And yes the brochures and manuals have all sorts of airbrushed drawings and photos that are not quite right.
Well Kraig - you got me on that one since I can't run out an look. Maybe some of the other guys on here will have a look at their mowing deck sub-frames with the deck raised and lowered, and then report back if the about the position of the cross-frame bar in the slot.