When I first began shaving around 1962, the Gillette Super Speed was available like the fusion is today. I purchased a Super Speed with the blue tip. It was a mild razor (as was my stubble). The regular (or medium) Super Speed did not have a colored tip, and the "heavy" had a dark red tip (actually, I recall it as "brown" but cannot explain why other than this was more than half a century ago).
A few years later, the Gillette Slim Adjustable razor was introduced. I bought two of them and rotated them for a number of years. It seems to me the blue tip was the equivalent of a 3 on the slim. This is the nature of an adjustable. A setting of 1 or 2 was milder than the blue tip, and a setting of 4 had a bit more blade exposure than did the blue tip. I tried the red tip Super Speed a few times (these razors were widely available and sold for maybe a $1.00), and it was the equivalent of a 7 on the slim, which was far too high of a setting for me.
The slim of course has a range of settings, and that range is obviously wider than what any one of the Super Speeds presented. It seems to me the slim went like this: 1-3 was "mild", 4-6 was "medium", and 7-9 was "heavy", with the these words (used by Gillette for marketing purposes with the Super Speed) referring to the nature of a person's beard. I had used the blue tip for several years before buying a slim and am certain that a setting of 3 was the equivalent of the blue tip. As for actual shaving results, it also seems to me the Merkur Progress is every bit as good as the slim, though it is a more aggressive razor. By that I mean a setting of 1 on the Progress is more aggressive than a setting of 1 on the slim, and that is because the slim has a wider range of settings. Both razors have a mid-point setting, equal to 1.5, 2.5, and so on. The slim has a 1-9 range of settings, the Progress 1-5. So, we see that the slim has a much wider range of settings, with 17 compared to the 9 of the Progress. The slim can thus be set higher or lower than the Progress.