An extending upper barrel, in addition to the pen/pencil combination, provided the owner with much greater flexibility and reduced the limitations imposed by the common fixed length full-sized desk pens and pencils of the day. One could now take a quality pencil/pen with them when they travelled as the overall length could be reduced by as much as 5 cm (2") when closed, making the writing implement much easier to tuck away in a coat pocket or travel bag.
I currently have a number of these gold combos in my collection from American makers, along with an interesting one from a British maker.
Albert Bagley - The inner barrel has Bagley's patent date imprinted on it "Patented Jan 1 1850". Gold, fluted design, with cut & polished green chalcedony in finial. 10 cm (3 7/8") when closed and 15 cm (5 7/8" when the barrel and pencil are extended.
Watherston & Son - This one is interesting for a few reasons. James Henderson Watherston and his son Edward James Watherston began their business together in 1864, in London. They advertised as goldsmiths, jewellers, and gold chain makers. Their adverts also stated that they were "Goldsmiths to H.R.H. the Prince of Wales" (Queen Victoria's son Albert Edward, who became King Edward VII following Victoria's death in 1901). Their business was quite well respected for the quality of their craftsmanship, however, I have yet to find any indication that they made writing implements of any sort. This suggests that they may have had their writing implements made by one or more of the reputable pencil-case makers of the day such as Sampson Mordan (the 1898 Mordan catalog includes an example of a similar extending upper barrel combo) and then branded them as their own.
Like the previous examples, this combo is gold, but unlike the more commonly seen fluted pattern, the barrel on this one is the barleycorn pattern. It is 9 cm (3.5") when closed and 13.6 cm (5 3/8") when extended.
There are many other examples of this style of combo from several makers, including Albert Bagley, Kurtz & Monaghan, John Mabie, and John Rauch.
Jonathan Veley has also documented these types of combos from his collection, and his blog provides a great deal of additional information related to the various American makers and their associated patents. Jon's blog can be found here - https://leadheadpencils.blogspot.com/2017/08/everybody-and-their-brother.html