Hi Matt 12,
There is a compatability issue between the Streamlined Euro Pin base and the Chinook Euro Pin universal pin.
If I remember correctly, the distance from the bottom surface of the Streamlined bases works prefectly with the North (and most of the others as the North "Hardcore" system was the original standard here along with Mistral/F2/Arrows), but if you try to use the Streamlined Extension on the Chinook versoin of the Euro pin, the placement of the groove that engages the locking clip (spring wire or a flat plate slider) is a little shorter, and the lock clip/slider will not engage.
You can "fix" this with a few strokes of a file across the bottom of the plastic mast base adapter to shorten the distance up to the lock clip/slider, but then your base will be a little loose on the std. North/Mistral/ most common Euro Pin bases.
Another fix (doesn't work on the new all SS Chinook base assemblies) is to simply get a North pin, a little Loc-Tite, and change the pin.
I've also run into a problem with some of the bases now being made in China for Starboard and others. They use a flat slider plate instead of a wire spring clip, and they do not radius the inside of the "jaws" so if your mast base pin has a radius, it won't engage as the flat vetical surfaces in the "jaws" of the slider plate won't go over the radius in the bottom of the groove on the pin.
Hope this helps,