DeepDiver writes:

So why is the o-ring there? Is it because true Bridgeport mills
have a larger through-hole in the spindle that allows the o-ring to
fit down inside the hole?

The Bridgeport J-head drawbar has a raised step there, not an O-ring, to
center the spacer bushing. The O-ring might just be a cheaper way to
approximate that shape.

The import mill-drill just has a straight bar with pinned hex head.

2. The RF-30 (and it's various "knock-offs") do not have a spindle
lock to hold the spindle from turning while tightening the drawbar. Do
Bridgeports and/or other larger mills have spindle locks? If not, how
do you sufficiently tighten the drawbar?

Bridgeports have a spindle brake.

On the mill-drill, I just hold on to the outside of the pulley to get
leverage.