I have had the chance to obtain real B737-200 throttle for my simulator. On them I can see only 2 detents. Once the throttle handles on iddle position it is possible to activate the reversers. So the first detent (the only one) is at about 3/4 of it's course and left 1/4 to full reverse... So I suppose they are differents from 737NG throttle. I know it is a little different from outside but do not know for the mecanism.
The -200 only had 2 detents as you have found. The first was idle, the second if you found it would have overboosted the engines (ie. exceeded max TOGA EPR). Max reverse was normally when the levers were vertically up and then trimmed towards the TOGA bug on the EPR gauge.
BUT...Day to day, the -200 reversers you would never use more than about 1.3 EPR (TOGA EPR was just over 2.1 (-15 engines)) as the buckets were so effective.