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Your Pin is not tack welded to the cover. Your pin is Stainless Steel, and your covers are aluminum squares (as far as I can tell from the photo). Some of the Sparcraft masts had covers that were tack welded to the pins, and if you took the screws out of the cover plate, and rotated and slightly pulled the cover the pin would come out with it. These also have a single hole drilled on the opposite side of the mast as the cover to allow you to insert any small long straight tool to push the pin out of the mast should it become dislodged from the cover plate.
Your cover plates cover a hole just large enough to allow you to put the pin in the mast. Generally there are a series of holes the pin goes through. From the cover inward. Hole through the mast side, Hole through the pin support plate (welded to the mast section itself), hole through the sheave, hole through the sheave separator (Generally either aluminum or phonalic canvas and resin board), hole through the opposite side sheave, hole through the opposite side support plate, and finally a smaller hole through the opposite side mast wall.
The small hole in the opposite side mast wall allows you to remove the pin, but keeps the pin captive between the cover plate and the mast wall on the opposite side. This way you don't have to have 2 cover plates, and 4 screws to hold the pin in, only one cover plate, and two screws on one side.
Does this make sense? I don't have a photo of this handy....
Guy