Mines could replace the torpedo load

Could fast torpedo boats carry mines instead of torpedoes?

Yes; fittings allowed boats to carry mines on either side in place of torpedoes when the mission called for minelaying.

Mines could be substituted for the normal torpedo load. The side fittings allowed a boat to carry a couple of mines on each side. This was not the universal configuration, because the choice depended on the assigned mission. The exchange reveals the operational flexibility hidden behind a class's standard weapons list.