John Gill Commentary Exodus 26:21

"and their forty sockets of silver; two sockets under one board, and two sockets under another board." — Exodus 26:21 (ASV)

And their forty sockets of silver
answerable to the twenty boards, for their two tenons to be placed in as in mortises: two sockets under one board, and two sockets under another board; and so under all the boards on the north side as on the south.