It appeared to me that these two dowels (arrowed) are holding them in.
To test this I hammered them out on the broken lock and sure enough, the barrel falls out easily.
Looking at the lock barrel it looks to me like at least one of the dowels is moulded into the barrel itself, making swapping these barrels almost impossible without attaching some kind of new dowel. The other thinner dowel that goes into the side is possibly removable but appears to be tapped into a hole making any ideas of disection almost impossible.
Conclusion: Don't bother approaching the car until you're prepared to don a new key or get creative with fiddly small metal pieces. I guess back in the day it was easy to get one ordered from the factory.
