Well thank you all for the warm welcome here
So I have confirmed that the screw used on the Baggage-Express is indeed DIFFERENT than the one on the OB Coach.
It has a countersink type head on it and it sits nicely inside the recessed portion of the 6-axle truck.
One thing I noticed is that the bolster hole on the 6-axle truck will not fit onto the OB bolster pin. You will need to take a bit of material off the bolster on the truck. Enough to get the truck to sit and rotate, but not so much that the baggage express screw can go all the way through!
I took a 6-axle truck and filed out the inside of the hole, and was able to get it to sit. The head of the baggage express screw is ~0.116", and I had to widen the hole in the truck to about 0.111" to get it to fit. That's pretty close to the size of the screw, so you really want to make sure you don't make it too big!
You will also need to use a washer of some kind to raise the car up a bit, otherwise the wheels will rub on the underframe.
I can send you some screws if you send me a PM with your address!