Good question. No idea.
The American documents on the British-captured U-570 go into detail on the TDC, but say nothing about the firing mechanism, except for the one on the UZO:
Quote:
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A small lever on the starboard side of the pedestal of the mounting was stated by the ship's gunner to be a torpedo firing lever for firing bow tubes from the bridge.
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http://www.uboatarchive.net/U-570ONIReport.htm
The British document also goes into detail about how the firing mechanisms work, but nothing I could find on how they were actually actuated.
I've looked around for pictures of the torpedo firing controls but couldn't find any on short notice. Hopefully someone will turn some up.