Charles Ellicott Commentary Exodus 26:19

"And thou shalt make forty sockets of silver under the twenty boards; two sockets under one board for its two tenons, and two sockets under another board for its two tenons." — Exodus 26:19 (ASV)

Forty sockets. Each “socket” was to receive one of the “tenons.” As there were twenty boards (Exodus 26:18), and two tenons to each board (Exodus 26:17), the sockets had to be forty.