Yes, towards the end of the war the boats stopped using external torpedo reserves (it was too dangerous to transfer torpedoes on the surface by that point), and some of the space under deck used for storing the torpedoes was cut away, which in theory meant less air was trapped under the deck and the boat's bow would go under much quicker, letting the sub dive faster.
In practice, as I recall, this actually made little difference, though for a boat its size the IXC was always a pretty quick diver. This, by the way, was done to many Type IX boats, except for the IXDs which continued to carry torpedoes externally.
The VIIC/41, by the way, is also often seen with little canisters on the front, which replaced the extra torpedo storage with inflatable lifeboat storage. A sign of the times after 1943!
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