I’ve not been through McConnell but in the Miller’s Checklist book, West says about Regency Decanters that “..there were revivals of the Regency style in the 1880s and 1930s. Often too bright in colour, they have neck rings that are moulded out of the body rather than applied...”. My feeling is the moulded neck rings makes the body date to at least around 1900.
The hollow stopper peg is a modern feature isn’t it? Not sure it matters as if the top of the peg sits almost 1/4 inch bellow the top the neck, I would say it is a replacement - regardless of how well it fits otherwise. Apart from the peg being low in the neck, aesthetically the bulls eye disk looks too close to the bottle rim to me.
It does look old though, sounds like you got some other interesting Decanters with it.