This actually gets a bit tricky, because you don't have a conventional string-through pickup. You actually have a cover plate from the 1st generation that was re-drilled to accommodate the poles from a 2nd-generation pickup.
Normally (in either version), the magnets would be held down by the top plate on either side of the coils:
They would not be visible in any of the pics you provided. However, it looks like the top plate has been trimmed and the whole thing pinned to the base plate instead of using the normal mounting screws. Normally, this would not leave room for magnets at all. I'm wondering if they snuck them into the middle of the pickup, in between the coils. If you have a spare piece of steel, I would poke various parts of the pickup and base plate to see if you can feel a magnetic attraction. If my suspicious are correct, you don't need any pickup parts at all (though the coils could always be in need of a rewind).
If you're looking for a plastic cover that hides the pickup, a steel this early wouldn't have come with it.
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Old Frets: the obscure side of vintage instruments.
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Vintage National Guitar Serial Numbers

National Lap Steel Serial Numbers

Hello, I recently acquired a Supro Lap Steel. C5414 with the letters FS impressed into the top of headstock following the SN. From what I can tell it is a 1938-1940 Avalon model. The light blue Supros were only made for a short period in the mid-'60s. Look for small metal plate nailed to the back of the cabinet - that is where the serial number is found.