About the artifact: as Alan stated, this is a safety clip of a torpedo pistol, maybe coming from a MkIIIa; it is shown in the image posted by Drew, see the attached images for reference.
About the context: this is kind of confusing, on that period HMS REGENT was deployed between the Ionian sea and the Adriatic sea in order to hunt for Italian ships tranferring troops to Greece.
In the same days the British SM5 operation was undergoing, meant to support the struggle of Malta by reinforcing the island with troops and supplies; a large British/Australian force comprising 2 battleships, 1 carrier, 1 heavy cruiser, 4 light cruisers and 11 destroyers sailed from Alexandria the 23rd of September.
In order to intercept the British force, the Royal Italian Navy deployed 4 battleships, 7 heavy cruisers, 4 light cruisers and 23 destroyers from Taranto the 29th of September, the two forces never engaged.
On 30th September 1940 HMS REGENT intercepted the Italian force in the Ionian (well far from Durazzo) sea and fired a salvo of 5 torpedoes, most likely against the battleships, without success.
Given the above, I cannot see any relation between the 30th of September and Durazzo. It was not uncommon for submarines (of every flag) to claim some successes that were actually never achieved, as after the attack the disengage was a priority due to the defender's destroyers depth charges hunt, because of this it might be understandable for HMS REGENT to claim a successfull attack on the 30th, but the Durazzo location is not.
HMS REGENT achieved success off Durazzo a few days later on the 9th of October, damaging the merchant ship SS ANTONIETTA COSTA later declared a total loss, interesting enough a few days earlier on the 4th October the SS ANTONIETTA COSTA collided with HMS RAINBOW which exploded.
As the engraving on the clip reports "Italian fleet", the episode is referring to the attack held on the 30th, the location Durazzo is a mistery, most likely a mistake that joined two different episodes of close days.
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The discrepancy between date and location might be explained by the fact that aboard a submarine, but also aboard surface vessels, fews knows the position fix with accuracy but the Officials, navigation and communication personnel.
The commemorative engraving was most likely produced by someone out of the "infarmation circle", which knew the date and the nature of the episode but not the precise location, somewhere between the Ionian and the Adriatic seas.
A confimed hit such as the one achieved on the 9th October brings details of the action to most of the crew, a close contact with a large enemy force followed by an engagement is defenitely something to be rememberd aboard although not crowned by success (to be also noted that this attack was the submarine's baptism of fire), the personnel that prepared the present might have thinked that the Italian fleet was intercepted the 30th somewhere near the Durazzo action of the 9th.