The small stainless caps pictured here clip to a groove in the hub, which is present on all 200/700/900 series, but you need to be using steel wheels in order for this groove to be exposed. Alloys are much thicker and cover the centre of the hub, and some or all of the grease cap.
There is a plastic cap that sits proud of the centre of the wheel, and fits over the grease cap in some applications. Original application was '79 model 260's, being a one year only part they are pretty thin on the ground, but if you can find some and they are the right diameter for the ATS wheels, they should do the job.
I can probably get a pic of some in a couple of days time.