4:3a Below the rim, figures of oxen encircled it, ten per cubit all the way around the Sea,

There were figurines/carvings of bulls/oxen all the way around the Sea, under/below its rim. There were ten bull/ox figurines/carvings in each forearm-measure.
-or-
Under the rim of the basin were likenesses of bulls/oxen, 10 likenesses every half meter. They went all the way around the basin.
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Decorations in the shape of bulls/oxen were molded into the basin just below the rim. There were ten bulls/oxen every foot and a half all the way around it.

4:3b cast in two rows as a part of the Sea.

The bull/oxen figurines/carvings were in two rows. They were part of the same form/mold from which the Sea was cast/molded.
-or-
There were 2 rows of cast metal/bronze bull/oxen likenesses, all part of the cast metal basin.

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