Match safe! I hadn't thought of that. In that case (no pun!), it might be from the 19th Century. Joshua Pusey invented book matches in 1879 because his match safe spoiled the appearance of his suit. Advertising appeared on book matches in 1897, and that greatly increased production. Lighter flints went into production in 1907. In WWI, I've read of soldiers using book matches and lighters but not stick matches. The Zippo, known for being windproof and reliable, came out in 1932.
In "The Sand Pebbles," a worker inside a steam engine was killed because a saboteur had removed the locking mechanism. The author had been an engineer on a similar vessel. If the danger was known, one would want a tamper-proof pin. Perhaps the groove described by the owner was made for a slotted plate to engage. Put the pin in, slide the plate on, and screw the plate down with tamper-proof screws. The ship's engineer would have the special screwdriver. Perhaps SE stands for Ship's Engineer. If it's solid, that's my guess!