It is a MT-2 integral shank. Link is close, but not the same one. Yes it came with a pin spanner. Both totally irrelevant to not being able to snug it without opening the top and holding the sheave. Yes it tis the old Delta as you show. New belts, well aligned, good lube on the bearings and it just has too little resistance. At least with a key, go around three times and you should never spin a bit. Seems like I do half the time with this one.
I see several Earl Diamond chucks (keyed); are they any good? Growing up, it was all Jacobs, now they seem to be Chinese garbage.