Two of the porcupine quill writing implements in my collection are pencils, while the third is a dip pen, and all three date to around 1880. The dip pen was made by Sampson Mordan (the sterling silver ferrule is stamped "S. Mordan & Co."). The overall length of the pen is 23 cm (9") and the quill measures a healthy 18 cm (7").
The smaller pencil was also made by Sampson Mordan ("S. Mordan & Co. is stamped on the cap). It is quite tiny compared to the dip pen; just 6.5 cm closed and 9 cm when fully extended (2.5"/3.5").
It is 8 cm long when closed and 10 cm long when extended (3.25"/4.0").
Porcupine quills have traditionally been used for a great variety of purposes and they are still widely used (there are many sources of both North American and African porcupine quills on the internet for those in need), but as far as I have been able to determine, their use as pen/pencil case barrels was fairly short-lived. I suspect that the quills simply did not stand up to the rigours of daily use and abuse.