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The 3D printing process for metals such as "Stainless Steel" can vary slightly with each print. These variations are very small but it can sometimes make the stem for the Cherry MX stem slightly too loose or slightly too tight.
Loose stems can be corrected by taking a small pair of pliers and lightly pinching the stem in all directions until it has narrowed enough to grip the Cherry MX switch correctly.
Tight stems are trickier to fix but can still be modified by using a flat head screwdriver to gently push out the inside of the stem.
Metal keycaps should hopefully fit your keyboard as soon as you get them, but if they need adjustment please proceed carefully so as not to damage your keycap.
If you are uncomfortable making these adjustments then the other materials may be a better option as these print and fit every time!