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hvsds
The sections are joined* by threaded nipples, so your leak is probably having to track down the screw threads and past the fibre washer.
Each section has a Left hand thread one side and Right hand thread on the other. So pairing a LH thread with a RH thread allows the nipple to draw two sections together as it's rotated.
Getting the plugs out of the top would show which end/sides are left/right thread. You can measure the thread size required and order some parts.
https://youtu.be/Nu1RJQGSrKk
https://www.castrads.com/uk/product-category/spares-accessories/
(*rare, but some rads just have a long tie rod right through, with Bushes to align the sections and seals )