I agree with what has been said here so far.
You definetely have a fine example of L20T3 in WALNUT circa 1989-1991. I own L20t3, L40T3, L80T3, and 2 pairs L100T3. My L100T3's both are walnut, the rest are oak. The Oak being the most rare being that they were only produced in 1989 in Oak veneer.
It is possible to sand these Braewood veneers with 400 grit paper by hand using a block. The veneer is so thin that any more coarse paper will burn through in a few stokes. Do not try to sand the edges. Only sand if absolutely necessary to remove surface scratches, it will make them feel like glass because there is no real wood grain, and all of the "grain" feel will disappear although appear to remain. The typical "suntan" or "bikini lines" the T3's suffer from is due to 2 factors. 1 is the veneer is not thick enough to hold a sufficient quantity of stain. It is a pigment and not a dye, so the stain sits on the top of the wood product, not inside the cells. The second most important reason is the stain used did not contain UV blocker, so the only T3's without the grill lines showing have been kept in a cave for all these years or never had the grills on. Far from JBL's best cabinet work here. When your oil finish soon becomes dull again, consider using wax to keep the shine and glow longer. I recommend a product called Briwax in walnut color in the OP's case, the cabinets appear to be in such good condition that any light scratches will be filled by the wax, and the color will fill out any existing finish irregularities due to aging. An alternative product that is easier to work with product is Howard's Feed N' Wax which will not correct color or provide as much protection as Briwax. To remove wax use mineral spirits and lots of rags.