Most S. rhombeus (in the home aquarium) turn dark grey in color, some black as adults. The eye is really the only distinctive part (deep blood red). As for "type" not sure what you mean by that because locality (where it was collected) is known only to the person taking it out of the water. The rest is selling point.
We don't know where that fish was collected, so we can't tell you.
Maybe the previous owner or store where you bought him could tell you - if not, you'll never know for sure...
As far as "diamond", "highback" etc. - you can give that fish every name you want to, as its just common names based on external features, without any value or meaning (as anyone could invent those names).