Haven't got the answer for you but I'm glad you've bought the subject up as I need to know as well .
1963 austin Cooper s mk1 1071
1966 Austin Cooper s mk1 1275
1968 Austin Cooper mk2 998
1962 Morris mini super 850
Porsche 997 turbo S
Ford transit van 280s.
I thought MK 1 racks didn't have the plug for the centralizing / locator pin.
The other way to check is to measure the travel of the rack in each direction which should be 1.75 " in old money.
I was pondering this question too. I have a few racks stored and didn't label them. A quick bit of research tells me that the Mark 1 rack had 15 teeth. I'm not sure how easy it is to count them without dismantling though. I also don't know if the mark 2 has more or less! My guess is more as the track rod ends on the mark 2 were closer to the hub and the rack has more travel.
When the mark 2 rack was introduced it had a plastic plug, on the opposite side to the pinion, which is removed to temporarily drop in a locator pin that locks the rack in the central position. When on the car this would be under the rubber grommet in the heel board. I don't know if this practice carried on in the later marks.
I haven't looked closely at my racks to see whether either of these identifying methods works! Maybe I'll have a look tomorrow.