Nice- did you take a .308 kit (woodcraft) and a bolt (PSI)kit and combine?
That doesn't look like the Woodcraft kit. It looks like a real cartridge with a drilled-out bullet, not a fake one.
I've actually been noodling building a pen like this for some time. The way that I've imagined it, I would cannibalize a kit from a screw-top capped pen (such as those Classic American kits that have been hanging around for years) for the coupler and center band. I'd drill the brass to accept the coupler and turn down the center band to fit inside the top tube. I'd probably have to frankenstein the tube so that it will accept the turned-down center band coupler and the bolt-action kit's top section.
Another way (and perhaps easier way) to do it is to use the tip coupler from a Patriot/Polaris kit. Turn down the little depth-stop bump and press it into the bolt-action kit's tube. Then drill and tap the brass to mate with the coupler's threads.