2 stage Desiccant Dryer Filter System question

I have a 2-stage Desiccant Dryer Filter system. The unit with the one quart capacity has a tube in the middle with a small filter on the end. Inside the tube is a rod with threads on the end.

When I removed the black collar ring and took the canister off, the small filter on the end of the rod was off.

I bought this second hand so I’m not sure if I’m missing something? I do not see where the threads screw into.
The small filter has an O-ring and a rubber connector about 1/4 wide, that appears to seal the tube to the end of the canister.
Is the o-ring and the collar replaceable?

Also the Desiccant refill I assume goes into the canister, mine did not have any inside.

Thanks for your time.