GP4 sprint rims:
First version: standard "box section" tubular rim (actually wider than it is deep, and the horizontal part is not flat), dark grey anodised, about 400g, with red label.
2nd version: same rim but with yellow and blue label.
3 rd version: same yellow and blue label but rim is a different shape (more square), heavier, and deeper like the HP MA40 but still less deep than a modern rim like the Open Pro.
The original shaped GP4 is just a normal sprint rim used with rim brakes, actual track rims do not have parallel sidewalls. if any.
... deeper sections and machined braking surfaces.
I would guess after the anodising on the sideall's worn off from braking, it might look like it's be machined.
The GP4 is actually a very heavy, bottom of the range rim, or to put a spin on it: bombproof, strong, good value for money.
Before the days of machined sidewalls, the braking surfaces were just the sidewalls, you don't actually need "machined braking surfaces", rims (and Mavic) have gone downhill since they were introduced as an extra feature so that they could raise prices.