No, that is the barrel catch mechanism as fitted by the Webley & Scott factory as part of the overal design (although it does have a threaded inlet for accomodating a silencer adaptor). The same attachment on the Eclipse Mk.II is/was much more aesthetic, however, as imo, the one as featured on this Eclipse Mk.I is as ugly as sin. I was not / am not alone in thinking this though, which was why it was so radically re-worked for the Mk.II ... I like the Mk.II, by far the better looking of the two.
EDIT: I hadn't scrolled through the advert far enough to see the silencer as detailed / asked about, so have offered some info on that at post # 19 below, although Baz has already admirably covered this at pistol # 18.
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