Many of the regulars here would probably be able to say if they are selling a MK5/MK10 with steel or brass turret retainers, but since the only way to find out is opening the first stage the average seller would not know the answer to that question.
If buying blind, I assume MK10 retainers to be steel and MK5 retainers to be brass. That will cover 95% of used MK5/MK10s out there.
Even some of the early 90s version MK5s with modern style yoke screw came from the factory with brass retainers, so it might be possible to find a few of the very earliest MK10s with brass retainer.
Glad I picked up a few steel replacements from VDH a few years ago. not sure where to source them now.