Arne,
I'm no expert but it is my impression that it was not uncommon to use solder as seen in your picture. I have done it myself on a Bucks County rifle. However, we don't know what happened to that gun over the past 200 years. It might have had a one-piece entry pipe that was damaged and in reparing the gun the repair was made by merely soldering on a replacement extension, which later fell off. Clearly, from the wear on the entry pipe, the gun has been used for a long time with the extension missing.
Best Regards,
JMC