The mount for the depth stop is definitely missing. I've got a couple of the variable speed PM drills from the 70's which have the same head castings, etc. If the mount was there, you'd see part of it immediately below the slot in the head castings.
If the OP's primary use is metalworking, the absence of the depth stop is of little consequence. In my own machine shop, I might use the stop once every few months to maintain uniform depth when countersinking a bunch of holes for flat head screws, but otherwise it remains largely unused. OTOH, a woodworker would likely use the stop much more often as a large number of holes for screws, dowels, etc are not thru holes.
The point though, isn't whether you need the stop or if you can cobble together a makeshift stop. The point is the machine is overpriced as is because it isn't complete. Reasonably priced drill presses are good sellers, and when one sits unsold for a month in a market the size of Dallas, no further proof is necessary that the price is too high and probably not negotiable to a reasonable level. The closer you look at the Powermatic, the more it reeks of a spray bomb "rebuild".
Don't know if others have noticed this, but I find it remarkable how often non-running or incomplete stuff is advertised by a seller who claims to be an expert on the device, but for some reason they don't put that expertise to work to enhance the value of what they're selling.
Car doesn't run, but the "experienced mechanic" seller knows it'll only take a set of plugs for it to run like showroom new.
Or a "30 year machinist" who's selling an incomplete drill press when any half-*** machinist could reproduce a professional appearing replacement depth assembly in a couple hours and add far more actual value than a half *** clean and spray job which automatically makes anyone suspicious if they're familiar with machinery and the common tricks used to hide problems.
If someone was peddling an airplane with no wings on CL, they'd likely claim 30 years astronaut experience and that it'll work fine without wings.