I have a Remington #1 Rolling Block that has a similar barrel setup! When I bought the gun it had a 20" half octagon barrel on it. It shot wonderfully, so I was happy, but it felt light, and aesthetically didn't look right to me.
I talked with John Taylor about what could be done to fix it, but retain the good shooting barrel. John came up with the idea to turn the last part of the barrel's diameter down, and make up a "bloop tube" that he would press over the turned down section. The bore on the extension is .40 caliber, and is 10" long; making it a 30" barrel.
Once it was polished out, and the whole barrel rust blued, the extension doesn't show. It looks like a .40 caliber barrel at the muzzle, and a .22LR at the chamber. Best part is it handles well, looks great, and shoots like it did before!
Nowhere near as good looking as your Winchester, but it's a fun gun!
